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