# Count of numbers of length N having prime numbers at odd indices and odd numbers at even indices

Given a number **N**, the task is to calculate the count of numbers of **length N** having **prime numbers** at **odd indices** and **odd numbers** at **even indices**.

**Example**:

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Input: N = 1Output: 5Explanation: All valid numbers length 1 are 1, 3, 5, 7, 9, here we have only 1 odd index, therefore we have 5 valid numbers.

Input: N = 2Output: 20Explanation:There are 20 valid numbers of length 2.

**Approach **: The problem can be solved with the help of combinatorics. The digits at odd indices have 4 choices and the digits at even indices have 5 choices.

Follow the steps to solve the problem:

- There are 5 choices for even indices (1, 3, 5, 7, 9 ) and 4 choices for odd indices (2, 3, 5, 7 ).
- For a number of length
**N,**there will be**N/2**odd indices and (**N/2 + N%2**) even indices. - So, the number of ways to fill N/2 odd indices are
**4**.^{N/2} - And the number of ways to fill even indices are
**5**.^{(N/2 + N%2)} - Hence, the total count of all the valid numbers will be
**4**.^{N/2 }* 5^{(N/2 + N%2)}

Below is the implementation of above approach:

## C++

`// c++ program to Count of numbers of length` `// N having prime numbers at odd indices and` `// odd numbers at even indices` `#include<bits/stdc++.h>` `using` `namespace` `std;` `// function to find total number of ways` `int` `find_Numb_ways(` `int` `n)` `{` ` ` `// No of odd indices in n-digit number` ` ` `int` `odd_indices = n/2;` ` ` ` ` `// No of even indices in n-digit number` ` ` `int` `even_indices = (n / 2) + (n % 2);` ` ` ` ` `// No of ways of arranging prime number` ` ` `// digits in odd indices` ` ` `int` `arr_odd = ` `pow` `(4, odd_indices);` ` ` ` ` `// No of ways of arranging odd number` ` ` `// digits in even indices` ` ` `int` `arr_even = ` `pow` `(5, even_indices);` ` ` ` ` `// returning the total number of ways` ` ` `return` `arr_odd * arr_even;` `}` `// drive code` `int` `main()` `{` ` ` `int` `n = 4;` ` ` `cout << find_Numb_ways(n) << endl;` ` ` `return` `0;` `}` `// This code is contributed by kondamrohan02.` |

## Java

`// Java program to Count of numbers of length` `// N having prime numbers at odd indices and` `// odd numbers at even indices` `import` `java.util.*;` `class` `GFG` `{` `// function to find total number of ways` `static` `int` `find_Numb_ways(` `int` `n)` `{` ` ` ` ` `// No of odd indices in n-digit number` ` ` `int` `odd_indices = n/` `2` `;` ` ` ` ` `// No of even indices in n-digit number` ` ` `int` `even_indices = (n / ` `2` `) + (n % ` `2` `);` ` ` ` ` `// No of ways of arranging prime number` ` ` `// digits in odd indices` ` ` `int` `arr_odd = (` `int` `)Math.pow(` `4` `, odd_indices);` ` ` ` ` `// No of ways of arranging odd number` ` ` `// digits in even indices` ` ` `int` `arr_even = (` `int` `)Math.pow(` `5` `, even_indices);` ` ` ` ` `// returning the total number of ways` ` ` `return` `arr_odd * arr_even;` `}` ` ` `// Driver Code` ` ` `public` `static` `void` `main(String[] args) {` ` ` `int` `n = ` `4` `;` ` ` `System.out.print(find_Numb_ways(n));` ` ` `}` `}` `// This code is contributed by code_hunt.` |

## Python3

`# python program for above approach` `def` `count(N):` ` ` `# No of odd indices in N-digit number` ` ` `odd_indices ` `=` `N` `/` `/` `2` ` ` `# No of even indices in N-digit number` ` ` `even_indices ` `=` `N` `/` `/` `2` `+` `N ` `%` `2` ` ` `# No of ways of arranging prime number` ` ` `# digits in odd indices` ` ` `arrange_odd ` `=` `4` `*` `*` `odd_indices` ` ` `# No of ways of arranging odd number` ` ` `# digits in even indices` ` ` `arrange_even ` `=` `5` `*` `*` `even_indices` ` ` `# returning the total number of ways` ` ` `return` `arrange_odd ` `*` `arrange_even` `# Driver code` `if` `__name__ ` `=` `=` `"__main__"` `:` ` ` `N ` `=` `4` ` ` `# calling the function` ` ` `print` `(count(N))` |

## C#

`// C# program to Count of numbers of length` `// N having prime numbers at odd indices and` `// odd numbers at even indices` `using` `System;` `using` `System.Collections.Generic;` `class` `GFG{` `// function to find total number of ways` `static` `int` `find_Numb_ways(` `int` `n)` `{` ` ` `// No of odd indices in n-digit number` ` ` `int` `odd_indices = n/2;` ` ` ` ` `// No of even indices in n-digit number` ` ` `int` `even_indices = (n / 2) + (n % 2);` ` ` ` ` `// No of ways of arranging prime number` ` ` `// digits in odd indices` ` ` `int` `arr_odd = (` `int` `)Math.Pow(4, odd_indices);` ` ` ` ` `// No of ways of arranging odd number` ` ` `// digits in even indices` ` ` `int` `arr_even = (` `int` `)Math.Pow(5, even_indices);` ` ` ` ` `// returning the total number of ways` ` ` `return` `arr_odd * arr_even;` `}` `// drive code` `public` `static` `void` `Main()` `{` ` ` `int` `n = 4;` ` ` `Console.Write(find_Numb_ways(n));` `}` `}` `// This code is contributed by SURENDRA_GANGWAR.` |

## Javascript

`<script>` `// Javascript program to Count of numbers of length` `// N having prime numbers at odd indices and` `// odd numbers at even indices` `// function to find total number of ways` `function` `find_Numb_ways(n)` `{` ` ` `// No of odd indices in n-digit number` ` ` `var` `odd_indices = n/2;` ` ` `// No of even indices in n-digit number` ` ` `var` `even_indices = (n / 2) + (n % 2);` ` ` ` ` `// No of ways of arranging prime number` ` ` `// digits in odd indices` ` ` `var` `arr_odd = Math.pow(4, odd_indices);` ` ` ` ` `// No of ways of arranging odd number` ` ` `// digits in even indices` ` ` `var` `arr_even = Math.pow(5, even_indices);` ` ` ` ` `// returning the total number of ways` ` ` `return` `arr_odd * arr_even;` `}` `// drive code` ` ` `var` `n = 4;` ` ` `document.write(find_Numb_ways(n));` `// This code is contributed by ipg2016107.` `</script>` |

**Output**

400

**Time Complexity**: O(1), (Constant time operations)**Auxiliary Space **: O(1), (No additional space required)