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