ACM比賽，acm

ACM比賽，acm

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

3 17 4

13
題目分析：此題的考點主要是數據的累加，在本題也可以說是階層的累加。

代碼：

#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;
}