#include <bits/stdc++.h>
int main() {
  long long int k, n, w, borrow;
  scanf("%lld %lld %lld", &k, &n, &w);
  long long int amount = (k * w * (w + 1) / 2);
  if (n < amount)
    borrow = amount - n;
  else
    borrow = 0;
  printf("%lld", borrow);
}
