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