#include <bits/stdc++.h>
using namespace std;
const int N = 1e6 + 10;
int main() {
  long long sum = 0;
  int k, n, w;
  cin >> k >> n >> w;
  for (int i = 1; i <= w; i++) sum += k * i;
  if (sum <= n)
    cout << "0";
  else {
    cout << sum - n;
  }
  getchar();
  getchar();
  return 0;
}
