#include <bits/stdc++.h>
using namespace std;
int main() {
  ios_base::sync_with_stdio(false);
  cin.tie(NULL);
  int k, n, w;
  cin >> k >> n >> w;
  int total_cost = 0;
  for (int i = 1; i <= w; ++i) {
    total_cost += i * k;
  }
  int borrow = total_cost - n;
  if (borrow < 0) borrow = 0;
  cout << borrow;
}
