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;
}