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