#include <bits/stdc++.h>
using namespace std;
int main() {
  int cost, earn, want, total = 0;
  cin >> cost >> earn >> want;
  for (int i = 1; i <= want; i++) {
    total += i * cost;
  }
  if (total <= earn) {
    cout << 0 << endl;
  } else {
    cout << total - earn << endl;
  }
  return 0;
}
