CF546A.Soldier and Bananas

A soldier wants to buy w bananas in the shop. He has to pay k dollars for the first banana, 2_k_ 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?

一名士兵想要在商店购买 $w$ 根香蕉。他购买第 1 根香蕉需支付 $k$ 美元，第 2 根需支付 $2k$ 美元，依此类推（即购买第 $i$ 根香蕉需支付 $i \cdot k$ 美元）。

他目前有 $n$ 美元。为了买下 $w$ 根香蕉，他需要向战友借多少美元？

The first line contains three positive integers k, n, w (1  ≤  k, w  ≤  1000, 0 ≤ n ≤ 109), the cost of the first banana, initial number of dollars the soldier has and number of bananas he wants.

第一行包含三个正整数 $k$、$n$、$w$（$1 \le k, w \le 1000$，$0 \le n \le 10^9$），分别表示第一个香蕉的价格、士兵最初拥有的美元数以及他想买的香蕉数量。

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.

输出一个整数——士兵必须向朋友借的钱数（单位：美元）。如果他不需要借钱，则输出 0。