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Number of ways to choose a pair containing an even and an odd number from 1 to N
• Last Updated : 20 May, 2021

Given a number N, the task is to find the number of pairs containing an even and an odd number from numbers between 1 and N inclusive.
Note: The order of numbers in the pair does not matter. That is (1, 2) and (2, 1) are the same.

Examples

```Input: N = 3
Output: 2
The pairs are (1, 2) and (2, 3).

Input: N = 6
Output: 9
The pairs are (1, 2), (1, 4), (1, 6), (2, 3),
(2, 5), (3, 4), (3, 6), (4, 5), (5, 6). ```

Approach: The number of ways to form the pairs is (Total number of Even numbers*Total number of Odd numbers).
Thus

1. if N is an even number of even numbers = number of odd numbers = N/2
2. if N is an odd number of even numbers = N/2 and the number of odd numbers = N/2+1

Below is the implementation of the above approach:

## C++

 `// C++ implementation of the above approach``#include ``using` `namespace` `std;` `// Driver code``int` `main()``{``  ``int` `N = 6;` `  ``int` `Even = N / 2 ;` `  ``int` `Odd = N - Even ;``  ` `  ``cout << Even * Odd ;``  ` `  ``return` `0;``  ``// This code is contributed``  ``// by ANKITRAI1``}`

## Java

 `// Java implementation of the above approach``import` `java.util.*;``import` `java.lang.*;``import` `java.io.*;``class` `GFG{` `// Driver code``public` `static` `void` `main(String args[])``{``  ``int` `N = ``6``;`` ` `  ``int` `Even = N / ``2` `;`` ` `  ``int` `Odd = N - Even ;``   ` `  ``System.out.println( Even * Odd );``   ` `}``}`

## Python3

 `# Python implementation of the above approach``N ``=` `6` ` ``# number of even numbers``Even ``=` `N``/``/``2` `# number of odd numbers``Odd ``=` `N``-``Even``print``(Even ``*` `Odd)`

## C#

 `// C# implementation of the``// above approach``using` `System;` `class` `GFG``{` `// Driver code``public` `static` `void` `Main()``{``    ``int` `N = 6;``    ` `    ``int` `Even = N / 2 ;``    ` `    ``int` `Odd = N - Even ;``        ` `    ``Console.WriteLine(Even * Odd);``}``}` `// This code is contributed``// by Akanksha Rai(Abby_akku)`

## PHP

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## Javascript

 ``
Output:
`9`

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