ACM士兵买香蕉，问他需要向他的朋友借多少钱

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Description

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?

Input

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.

Output

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.

Sample Input

Input

3 17 4

Output

13

解题思路：首先确定循环结束条件是士兵买完香蕉，给定一个w表示士兵要买的香蕉数，每循环一次w就减少一个，直到被减少到0时就结束循环。用s表示士兵要给的钱数，开始时给s赋值为0，每循环一次s就被更新一次，且每次s都在原来的基础上加上一个k*w.

最后将士兵所带的钱与士兵所要给出的做一个比较，如果士兵所带的钱是足够的，他就不需要向他的朋友借钱（即输出0）；否则士兵就需要向他的朋友借钱，这时输出他需要向他朋友借的钱数。

程序代码：

#include <iostream>

using namespace std;

int main ()

{

      int k,n,w,s=0,x=0;

      cin>>k>>n>>w;

      while(w!=0)

      {

            s=s+k*w;

            w--;

      }

      if(s>n)

      {

            x=s-n;

          cout<<x<<endl;

      }

      else 

            cout<<x<<endl;

      return 0;

}

ACM士兵买香蕉，问他需要向他的朋友借多少钱

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原文地址：http://www.cnblogs.com/xinxiangqing/p/4655500.html