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