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