#include <bits/stdc++.h>
using namespace std;
int main(void) {
  long long k, n, w;
  cin >> k >> n >> w;
  long long x = w * (w + 1) * k / 2 - n;
  if (x <= 0)
    cout << 0 << endl;
  else
    cout << x << endl;
  return 0;
}
