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