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