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