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