SOLUTION OF URI 1921

//    Solution of  1921 Guilherme and His Kites
//   (Mahmud_Jamil) BRUR




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

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