#include <bits/stdc++.h>
using namespace std;
int main() {
  long double k, c, n;
  cin >> k >> c >> n;
  long double sum = (n / 2) * (2 * k + (n - 1) * k);
  if (sum <= c)
    cout << "0";
  else
    cout << (long long int)(sum - c);
}
