#include <bits/stdc++.h>
using namespace std;
int main() {
  int k, w, n;
  cin >> k >> n >> w;
  int sum = 0;
  for (int i = 1; i <= w; i++) {
    sum = sum + i * k;
  }
  if (n > sum)
    cout << 0;
  else
    cout << abs(n - sum);
}
