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