#include <bits/stdc++.h>
using namespace std;
signed main() {
  ios_base::sync_with_stdio(false);
  cin.tie(0);
  cout.tie(0);
  ;
  long long k, n, w;
  cin >> k >> n >> w;
  long long req = ((w * (w + 1)) / 2) * k;
  req = req - n;
  if (req > 0)
    cout << req;
  else
    cout << "0";
}
