#include <bits/stdc++.h>
using namespace std;
int main() {
  int k, n, w;
  cin >> k >> n >> w;
  int x = k * (((w) * (w + 1)) / 2);
  if ((x - n) < 0)
    cout << "0" << endl;
  else
    cout << x - n << endl;
  return 0;
}
