虎哥准备买w个香蕉，他需要支付k元买第一个香蕉，支付2k元买第二个香蕉，...支付i*k元买第i个香蕉。

现在虎哥共有n元，他需要买w个香蕉，请问他还至少需要向朋友借多少钱？

A soldier wants to buy w bananas in the shop. He has to pay k dollars for the first banana, 2k dollars for the second one and so on (in other words, he has to pay i·k dollars for the i-th banana).

He has n dollars. How many dollars does he have to borrow from his friend soldier to buy w bananas?

三个整数k,n,w(1<=k,w<=1000, 0<=n<=1e9)。

The first line contains three positive integers k, n, w (1  ≤ k, w ≤  1000, 0 ≤ n ≤ 10^9), the cost of the first banana, initial number of dollars the soldier has and number of bananas he wants.

一个整数，表示虎哥所需借的钱数，如果不需要借钱，则输出0。

Output one integer — the amount of dollars that the soldier must borrow from his friend. If he doesn't have to borrow money, output 0.

天梯赛#4A