#include <bits/stdc++.h>
int main(void) {
  int b_cost, money, b_number;
  scanf("%d %d %d", &b_cost, &money, &b_number);
  int total_cost;
  int loan = 0;
  total_cost = b_cost;
  for (int i = 2; i <= b_number; i++) {
    total_cost += i * b_cost;
  }
  if (money < total_cost) {
    loan = total_cost - money;
  }
  printf("%d", loan);
  return 0;
}
