#include <iostream>
using namespace std;

int main() {
  int k, n, total_cost;
  total_cost = 0;
  long w;
  cin >> k >> n >> w;
  for (int i = 1; i < w + 1; i++) {
    total_cost += i * k;
  }
  total_cost -= n;
  if (total_cost < 0) {
    total_cost = 0;
  }
  cout << total_cost << endl;
}