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