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