#include <bits/stdc++.h>
using namespace std;
int main() {
  int n, m, k;
  cin >> n >> m >> k;
  long long int cost = 0;
  for (int i = 1; i <= k; i++) cost += (i * n);
  if (cost - m >= 0)
    cout << (cost - m) << endl;
  else
    puts("0");
}
