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