# Number of ways to choose a pair containing an even and an odd number from 1 to N

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: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).
```

## Recommended: Please try your approach on {IDE} first, before moving on to the solution.

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

Thus

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

 ` `

Output:

```9
```

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