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