There are **N** persons in a room. Find the maximum number of Handshakes possible. Given the fact that any two persons shake hand exactly once.

**Examples :**

Input : N = 2 Output : 1. There are only 2 persons in the room. 1 handshake take place. Input : N = 10 Output : 45.

To maximize the number of handshakes, each person should shake hand with every other person in the room. For the first person, there would be N-1 handshakes. For second person there would N-1 person available but he had already shaken hand with the first person. So there would be N-2 handshakes and so on.

So, Total number of handshake = N-1 + N-2 +….+ 1 + 0, which is equivalent to ((N-1)*N)/2

(using the formula of sum of first N natural number).

Below is the implementation of this problem.

## C++

`// C++ program to find maximum number of ` `// handshakes. ` `#include<bits/stdc++.h> ` `using` `namespace` `std; ` ` ` `// Calculating the maximum number of handshake ` `// using derived formula. ` `int` `maxHandshake(` `int` `n) ` `{ ` ` ` `return` `(n * (n - 1))/ 2; ` `} ` ` ` `// Driver Code ` `int` `main() ` `{ ` ` ` `int` `n = 10; ` ` ` `cout << maxHandshake(n) <<endl; ` ` ` ` ` `return` `0; ` `} ` |

## Java

`// Java program to find maximum number of ` `// handshakes. ` ` ` `class` `GFG ` `{ ` ` ` `// Calculating the maximum number of ` ` ` `// handshake using derived formula. ` ` ` `static` `int` `maxHandshake(` `int` `n) ` ` ` `{ ` ` ` `return` `(n * (n - ` `1` `)) / ` `2` `; ` ` ` `} ` ` ` ` ` ` ` `// Driver code ` ` ` `public` `static` `void` `main (String[] args) ` ` ` `{ ` ` ` `int` `n = ` `10` `; ` ` ` `System.out.println( maxHandshake(n)); ` ` ` `} ` `} ` ` ` `// This code is contributed by Anant Agarwal. ` |

## Python3

`# Python3 program to find maximum ` `# number of handshakes. ` ` ` `# Calculating the maximum number ` `# of handshake using derived formula. ` `def` `maxHandshake(n): ` ` ` ` ` `return` `int` `((n ` `*` `(n ` `-` `1` `)) ` `/` `2` `) ` ` ` `# Driver Code ` `n ` `=` `10` `print` `(maxHandshake(n)) ` ` ` `# This code is contributed by Smitha Dinesh Semwal. ` |

## C#

`// C# program to find maximum number of ` `// handshakes. ` `using` `System; ` ` ` `class` `GFG ` `{ ` ` ` `// Calculating the maximum number of ` ` ` `// handshake using derived formula. ` ` ` `static` `int` `maxHandshake(` `int` `n) ` ` ` `{ ` ` ` `return` `(n * (n - 1)) / 2; ` ` ` `} ` ` ` ` ` ` ` `// Driver code ` ` ` `public` `static` `void` `Main () ` ` ` `{ ` ` ` `int` `n = 10; ` ` ` `Console.Write( maxHandshake(n)); ` ` ` `} ` `} ` ` ` `// This code is contributed by nitin mittal. ` |

## PHP

`<?php ` `// PHP program to ` `// find maximum number ` `// of handshakes. ` ` ` `// Calculating the maximum ` `// number of handshake ` `// using derived formula. ` `function` `maxHandshake(` `$n` `) ` `{ ` ` ` `return` `(` `$n` `* (` `$n` `- 1))/ 2; ` `} ` ` ` `// Driver Code ` `$n` `= 10; ` `echo` `maxHandshake(` `$n` `); ` ` ` `// This code is contributed by anuj_67. ` `?> ` |

Output:

45

**Time Complexity : **O(1)

This article is contributed by **Anuj Chauhan(anuj0503)**. If you like GeeksforGeeks and would like to contribute, you can also write an article using contribute.geeksforgeeks.org or mail your article to contribute@geeksforgeeks.org. See your article appearing on the GeeksforGeeks main page and help other Geeks.

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