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