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