# Count of all possible numbers not exceeding M having suffix N

Given two positive integers N and M, the task is to find the count of all possible numbers in the range [1, M], having suffix as N.

Examples:

Input: N = 5, M = 15
Output:
Explanation: Only numbers satisfying the conditions are {5, 15}.
Input: N = 25, M = 4500
Output : 45

Naive Approach: The simplest approach is to traverse all integers in the range [1, M] and check if the suffix is N or not.
Time Complexity: O(M)
Auxiliary Space: O(1)

Efficient Approach: To optimize the above approach, following observation needs to be made:

Let N = 5 and M = 100
The Suffix numbers are 5, 15, 25, 35…95, which forms an Arithmetic Progression with
first term = 5, last term = 95, common difference = Base of N (eg: 6 has base 10, 45 has base 100 which is nothing but the exponentiation of the form 10digitsOf(N), where digitsOf(N) = no. of digits present in N.

Therefore, in order to calculate the count of possible numbers in the range [1, M], the following expression needs to be evaluated:

Count of numbers = Number of terms in the series = (tn – a)/d + 1 , where
tn is the last term of the sequence, a is the first term of the sequence, d is the common difference = (ti+1 – ti), i = 1, 2, 3…n-1

Below is the implementation of the above approach:

## C++

 `// C++ Program to implement` `// the above approach` `#include `   `using` `namespace` `std;`   `// Function to count the` `// no. of digits of N` `int` `digitsOf(``int` `num)` `{` `    ``return` `to_string(num).size();` `}`   `// Function to count all possible` `// numbers having Suffix as N` `int` `count(``int` `a, ``int` `tn)` `{` `    ``// Difference of the A.P` `    ``int` `diff = ``pow``(10, digitsOf(a));`   `    ``// Count of the number of terms` `    ``return` `((tn - a) / diff) + 1;` `}`   `// Driver Code` `int` `main()` `{`   `    ``int` `n, m;` `    ``n = 25, m = 4500;` `    ``cout << count(n, m);`   `    ``return` `0;` `}`

## Java

 `// Java program to implement ` `// the above approach ` `import` `java.util.*;`   `class` `GFG{`   `// Function to count the` `// no. of digits of N` `static` `int` `digitsOf(``int` `num)` `{` `    ``return` `Integer.toString(num).length();` `}`   `// Function to count all possible` `// numbers having Suffix as N` `static` `int` `count(``int` `a, ``int` `tn)` `{` `    `  `    ``// Difference of the A.P` `    ``int` `diff = (``int``)Math.pow(``10``, digitsOf(a));`   `    ``// Count of the number of terms` `    ``return` `((tn - a) / diff) + ``1``;` `}`   `// Driver code` `public` `static` `void` `main (String[] args)` `{` `    ``int` `n = ``25``, m = ``4500``;` `    `  `    ``System.out.println(count(n, m));` `}` `}`   `// This code is contributed by offbeat`

## Python3

 `# Python3 program to implement ` `# the above approach `   `# Function to count the` `# no. of digits of N` `def` `digitsOf(num):` `    ``return` `len``(``str``(num));`   `# Function to count all possible` `# numbers having Suffix as N` `def` `count(a, tn):`   `    ``# Difference of the A.P` `    ``diff ``=` `int``(``pow``(``10``, digitsOf(a)));`   `    ``# Count of the number of terms` `    ``return` `((tn ``-` `a) ``/` `diff) ``+` `1``;`   `# Driver code` `if` `__name__ ``=``=` `'__main__'``:` `    ``n ``=` `25``; m ``=` `4500``;`   `    ``print``(``int``(count(n, m)));`   `# This code is contributed by sapnasingh4991`

## C#

 `// C# program to implement ` `// the above approach ` `using` `System;`   `class` `GFG{`   `// Function to count the` `// no. of digits of N` `static` `int` `digitsOf(``int` `num)` `{` `    ``return` `num.ToString().Length;` `}`   `// Function to count all possible` `// numbers having Suffix as N` `static` `int` `count(``int` `a, ``int` `tn)` `{` `    `  `    ``// Difference of the A.P` `    ``int` `diff = (``int``)Math.Pow(10, digitsOf(a));`   `    ``// Count of the number of terms` `    ``return` `((tn - a) / diff) + 1;` `}`   `// Driver code` `public` `static` `void` `Main(String[] args)` `{` `    ``int` `n = 25, m = 4500;` `    `  `    ``Console.WriteLine(count(n, m));` `}` `}`   `// This code is contributed by PrinciRaj1992 `

Output:

```45

```

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

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