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

• Difficulty Level : Expert
• Last Updated : 06 Sep, 2021

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 = 1
Output:   5
Explanation : 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 = 2
Output:  20
Explanation: 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 4N/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``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

 ``
Output
`400`

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

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