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：题目意思是买w根香蕉，第一根k元，第二根2k元，以此类推，第i根i*k元，这个人手中有n元，则他买w根香蕉需要向朋友借多少元钱。解这个题目首先应注意的香蕉的总钱数在计算之前要赋值0，并且在计算完成后要判断总钱数与他自己所拥有的钱哪个比较大，再用大的减去小的得出结果。
程序代码：
#include<iostream>
using namespace std;
int main()
{
int k,n,w;
cin>>k>>n>>w;
int sum=0;
for(int i=1;i<=w;i++)
{
sum=sum+i*k;
}
if(sum>n)
cout<<sum-n<<endl;
else cout<<"0"<<endl;
return 0;
}