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