2015 HUAS Provincial Select Contest #1 A

题目：

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.

题目大意：第i个香蕉的价格是ik元，求总共的价钱

算法用等差数列的求和公式：（1+i）*i/2；

代码：

#include<iostream>
using namespace std;
int main()
{
    int k,w,n,sum,b,price;
    while(cin>>k>>n>>w)
    {
        if(k>=1&&w<=1000&&n>=0&&n<=1000000000)
    {
        sum=(1+w)*w/2;
        price=sum*k;
        if(price>n)
        {
            b=price-n;
            printf("%d\n",b);    
        }
        else printf("0\n");
    }
    }
    return 0;
}