Handshakes All Around
N people each shake hands once with everyone else. How many handshakes?
Input: one integer N.
Output: N(N−1)/2.
Constraints:
- 1 ≤ N ≤ 109
Sample Tests
Input 1
3
Output 1
3
Input 2
1
Output 2
0
Discussion (opens after you solve — no spoilers before)
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