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