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