#include <bits/stdc++.h>
int main() {
  unsigned k, n, w;
  scanf("%u %u %u", &k, &n, &w);
  int cost = k * ((w * (w + 1)) / 2);
  if (n >= cost)
    putchar('0');
  else
    printf("%d", cost - n);
  return 0;
}
