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