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