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