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
题目分析：此题的考点主要是数据的累加，在本题也可以说是阶层的累加。
代码：
#include <cstdio>
#include <iostream>
using namespace std;
int main()
{
int k,n,w,sum=0;
cin>>k>>n>>w;
for(int i=1;i<=w;i++)
sum+=i*k;
if((sum-n)<0)
cout<<0;
else cout<<sum-n;
return 0;
}