#include <bits/stdc++.h>
int main() {
  double k, n, w;
  scanf("%lf %lf %lf", &k, &n, &w);
  double total = (w / 2) * (2 + (w - 1));
  total *= k;
  if (total > n)
    printf("%.0lf\n", total - n);
  else
    printf("0\n");
}
