Find the maximum number of handshakes

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.  

Output: 

45

Time Complexity : O(1)