# Maximum number made up of distinct digits whose sum is equal to N

Given a positive integer **N**, the task is to find the largest positive number made up of distinct digits having the sum of its digits equal to **N**. If no such number exists, print **“-1”**.

**Examples:**

Input:N = 25Output:98710Explanation:

The number 98710 is the largest number that contains only unique digits and the sum of its digits (9 + 8 + 7 + 1 + 0) is N(= 25).

Input:N = 50Output:-1

**Approach:** The given problem can be solved based on the following observations:

**If the value of N is at least 45:**The required result is**-1**as the largest number that can be made using non-repeating digits is**9876543210**, whose sum of digits is**45**.**For all other values of N:**To get the largest number, start from the largest digit i.e.,**9**, and keep decrementing it and adding it to the required number. The required number must contain a**0**at the end of it as it increases the value without changing the sum of digits.

Follow the steps below to solve the problem:

- If the given number
**N**is greater than**45**, then print**“-1”**. - Otherwise, perform the following steps:
- Initialize a variable, say
**num**as**0**to store the required result, and a variable, say**digit**, as**9**. - Iterate a loop until the values of
**N**and**digit**are positive- If the value of a
**digit**is**at most N**, then multiply**num**by**10**and then add the value of**digit**to**num**and decrement the value of**N**by**digit**. - Also, decrement the value of the
**digit**by**1**.

- If the value of a
- Multiply the variable
**num**by**10**and append digit**0**at the end.

- Initialize a variable, say
- After completing the above steps, print the value of
**num**as the resultant number.

Below is the implementation of the above approach:

## C++

`// C++ program for the above approach` `#include <bits/stdc++.h>` `using` `namespace` `std;` `// Function to find the largest positive` `// number made up of distinct digits` `// having the sum of its digits as N` `long` `largestNumber(` `int` `N)` `{` ` ` `// If given number is greater` ` ` `// than 45, print -1` ` ` `if` `(N > 45)` ` ` `return` `-1;` ` ` `// Store the required number and` ` ` `// the digit to be considered` ` ` `int` `num = 0, digit = 9;` ` ` `// Loop until N > 0 and digit > 0` ` ` `while` `(N > 0 && digit > 0) {` ` ` `// If the current digit is` ` ` `// at most N then, add it` ` ` `// to number num` ` ` `if` `(digit <= N) {` ` ` `// Update the value of` ` ` `// num` ` ` `num *= 10;` ` ` `num += digit;` ` ` `// Decrement N by digit` ` ` `N -= digit;` ` ` `}` ` ` `// Consider the next lower` ` ` `// digit` ` ` `digit -= 1;` ` ` `}` ` ` `// Add 0 at the end and return` ` ` `// the number num` ` ` `return` `num * 10;` `}` `// Driver Code` `int` `main()` `{` ` ` `int` `N = 25;` ` ` `cout << largestNumber(N);` ` ` `return` `0;` `}` |

## Java

`// Java program for the above approach` `import` `java.io.*;` `import` `java.util.*;` `class` `GFG{` `// Function to find the largest positive` `// number made up of distinct digits` `// having the sum of its digits as N` `static` `long` `largestNumber(` `int` `N)` `{` ` ` ` ` `// If given number is greater` ` ` `// than 45, print -1` ` ` `if` `(N > ` `45` `)` ` ` `return` `-` `1` `;` ` ` `// Store the required number and` ` ` `// the digit to be considered` ` ` `int` `num = ` `0` `, digit = ` `9` `;` ` ` `// Loop until N > 0 and digit > 0` ` ` `while` `(N > ` `0` `&& digit > ` `0` `)` ` ` `{` ` ` ` ` `// If the current digit is` ` ` `// at most N then, add it` ` ` `// to number num` ` ` `if` `(digit <= N)` ` ` `{` ` ` ` ` `// Update the value of` ` ` `// num` ` ` `num *= ` `10` `;` ` ` `num += digit;` ` ` `// Decrement N by digit` ` ` `N -= digit;` ` ` `}` ` ` `// Consider the next lower` ` ` `// digit` ` ` `digit -= ` `1` `;` ` ` `}` ` ` `// Add 0 at the end and return` ` ` `// the number num` ` ` `return` `num * ` `10` `;` `}` `// Driver Code` `public` `static` `void` `main (String[] args)` `{` ` ` `int` `N = ` `25` `;` ` ` ` ` `System.out.print(largestNumber(N));` `}` `}` `// This code is contributed by sanjoy_62` |

## Python3

`# Python program for the above approach` `# Function to find the largest positive` `# number made up of distinct digits` `# having the sum of its digits as N` `def` `largestNumber(N):` ` ` ` ` `# If given number is greater` ` ` `# than 45, print -1` ` ` `if` `(N > ` `45` `):` ` ` `return` `-` `1` ` ` ` ` `# Store the required number and` ` ` `# the digit to be considered` ` ` `num ` `=` `0` ` ` `digit ` `=` `9` ` ` ` ` `# Loop until N > 0 and digit > 0` ` ` `while` `(N > ` `0` `and` `digit > ` `0` `):` ` ` ` ` `# If the current digit is` ` ` `# at most N then, add it` ` ` `# to number num` ` ` `if` `(digit <` `=` `N):` ` ` ` ` `# Update the value of` ` ` `# num` ` ` `num ` `*` `=` `10` ` ` `num ` `+` `=` `digit` ` ` ` ` `# Decrement N by digit` ` ` `N ` `-` `=` `digit` ` ` ` ` `# Consider the next lower` ` ` `# digit` ` ` `digit ` `-` `=` `1` ` ` ` ` `# Add 0 at the end and return` ` ` `# the number num` ` ` `return` `num ` `*` `10` `# Driver Code` `N ` `=` `25` `print` `(largestNumber(N))` `# This code is contributed by avanitrachhadiya2155` |

## C#

`// C# program for the above approach` `using` `System;` `class` `GFG{` ` ` `// Function to find the largest positive` `// number made up of distinct digits` `// having the sum of its digits as N` `static` `long` `largestNumber(` `int` `N)` `{` ` ` ` ` `// If given number is greater` ` ` `// than 45, print -1` ` ` `if` `(N > 45)` ` ` `return` `-1;` ` ` `// Store the required number and` ` ` `// the digit to be considered` ` ` `int` `num = 0, digit = 9;` ` ` `// Loop until N > 0 and digit > 0` ` ` `while` `(N > 0 && digit > 0)` ` ` `{` ` ` ` ` `// If the current digit is` ` ` `// at most N then, add it` ` ` `// to number num` ` ` `if` `(digit <= N)` ` ` `{` ` ` ` ` `// Update the value of` ` ` `// num` ` ` `num *= 10;` ` ` `num += digit;` ` ` `// Decrement N by digit` ` ` `N -= digit;` ` ` `}` ` ` `// Consider the next lower` ` ` `// digit` ` ` `digit -= 1;` ` ` `}` ` ` `// Add 0 at the end and return` ` ` `// the number num` ` ` `return` `num * 10;` `}` `// Driver Code` `public` `static` `void` `Main()` `{` ` ` `int` `N = 25;` ` ` ` ` `Console.Write(largestNumber(N));` `}` `}` `// This code is contributed by ukasp` |

## Javascript

`<script>` `// JavaScript program for the above approach` `// Function to find the largest positive` `// number made up of distinct digits` `// having the sum of its digits as N` `function` `largestNumber(N)` `{` ` ` ` ` `// If given number is greater` ` ` `// than 45, print -1` ` ` `if` `(N > 45)` ` ` `return` `-1;` ` ` `// Store the required number and` ` ` `// the digit to be considered` ` ` `let num = 0, digit = 9;` ` ` `// Loop until N > 0 and digit > 0` ` ` `while` `(N > 0 && digit > 0)` ` ` `{` ` ` ` ` `// If the current digit is` ` ` `// at most N then, add it` ` ` `// to number num` ` ` `if` `(digit <= N)` ` ` `{` ` ` ` ` `// Update the value of` ` ` `// num` ` ` `num *= 10;` ` ` `num += digit;` ` ` `// Decrement N by digit` ` ` `N -= digit;` ` ` `}` ` ` `// Consider the next lower` ` ` `// digit` ` ` `digit -= 1;` ` ` `}` ` ` `// Add 0 at the end and return` ` ` `// the number num` ` ` `return` `num * 10;` `}` `// Driver Code` ` ` `let N = 25;` ` ` ` ` `document.write(largestNumber(N));` `</script>` |

**Output:**

98710

**Time Complexity:** O(1)**Auxiliary Space:** O(1)