BUPT-homework/pset3/3-sum.c

#include <stdio.h>

int main(void) {
  // Initialize variables and get input
  long long sum = 0;
  long long n;
  scanf("%lld", &n);

  // if n is smaller than 3
  if (n < 3) {
    printf("0\n");
    return 0;
  }

  // Iterate through all
  for (long long i = 3; i <= n; i++) {
    sum += i * (i - 1) * (i - 2);
  }

  // Print the result
  printf("%lld\n", sum);

  return 0;
}