#include <bits/stdc++.h>
using namespace std;
int main() {
  ios::sync_with_stdio(0);
  cin.tie(0);
  int k, w, n;
  int numBorrow = 0;
  cin >> k;
  cin >> w;
  cin >> n;
  int total = k * (n * (n + 1) / 2);
  if (total > w) {
    numBorrow = total - w;
  }
  cout << numBorrow << "\n";
}
