# Count ways to express a number as sum of exactly two numbers

Given a positive integer N. The task is to find the number of ways in which you can express N as a sum of exactly two numbers A and B (N = A + B) where A > 0, B > 0 and B > A.

Examples:

```Input: N = 8
Output: 3
Explanation:
N = 8 can be expressed as (1, 7), (2, 6), (3, 5)

Input: N = 14
Output: 6
```

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

Approach:

• An observation here is that for every number N if we take a number A which is less than N/2 then, there must be a number B which is greater than N/2 and A + B = N.
• This leads to a simple solution of finding the count of numbers for either B or A. Hence the floor value of (N-1)/2 will lead to the solution.

## C++

 `// C++ program to Count ways to  ` `// express a number as sum of  ` `// two numbers. ` `#include ` `using` `namespace` `std; ` ` `  `// Function returns the count  ` `// of ways express a number  ` `// as sum of two numbers. ` `int` `CountWays(``int` `n) ` `{ ` `    ``int` `ans = (n - 1) / 2; ` `     `  `    ``return` `ans; ` `} ` ` `  `// Driver code ` `int` `main() ` `{ ` `   ``int` `N = 8; ` `    `  `    ``cout << CountWays(N); ` `} `

## Java

 `// Java program to count ways to  ` `// express a number as sum of  ` `// two numbers. ` `class` `GFG{ ` ` `  `// Function returns the count  ` `// of ways express a number  ` `// as sum of two numbers. ` `static` `int` `CountWays(``int` `n) ` `{ ` `    ``int` `ans = (n - ``1``) / ``2``; ` `     `  `    ``return` `ans; ` `} ` ` `  `// Driver code ` `public` `static` `void` `main(String[] args) ` `{ ` `    ``int` `N = ``8``; ` `    ``System.out.print(CountWays(N)); ` `} ` `} ` ` `  `// This code is contributed by Rajput-Ji `

## Python3

 `# Python3 program to Count ways to  ` `# express a number as sum of  ` `# two numbers.  ` ` `  `# Function returns the count  ` `# of ways express a number  ` `# as sum of two numbers.  ` `def` `CountWays(n) : ` `    ``ans ``=` `(n ``-` `1``) ``/``/` `2` `    ``return` `ans ` ` `  `# Driver code  ` `N ``=` `8` `print``(CountWays(N)) ` ` `  `# This code is contributed by Sanjit_Prasad `

## C#

 `// C# program to count ways to  ` `// express a number as sum of  ` `// two numbers. ` `using` `System; ` `class` `GFG{ ` ` `  `// Function returns the count  ` `// of ways express a number  ` `// as sum of two numbers. ` `static` `int` `CountWays(``int` `n) ` `{ ` `    ``int` `ans = (n - 1) / 2; ` `     `  `    ``return` `ans; ` `} ` ` `  `// Driver code ` `public` `static` `void` `Main() ` `{ ` `    ``int` `N = 8; ` `    ``Console.Write(CountWays(N)); ` `} ` `} ` ` `  `// This code is contributed by Code_Mech `

Output:

```3
```

Time complexity: O(N)

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