Solution of URI 1921 :: Guilherme and His Kites

Before seeing the solution make sure that you tried enough. Don’t paste the whole code, just find out the logic. If you stuck in trouble, just inform me on comment.

/**Bismillahir Rahmanir Rahim.**/

#include <stdio.h>
int main() {

    long long int a,n;
    scanf("%lld", &n);
    a=(n*(n-3))/2;
    printf("%lld\n",a);
    return 0;
}