#include <bits/stdc++.h>
using namespace std;
int main() {
  ios_base::sync_with_stdio(false);
  int k, n, w;
  cin >> k >> n >> w;
  long long t = k * (w) * (w + 1) / 2, h = 0;
  cout << max(h, t - n) << endl;
}
