A

A. Soldier and Bananas
time limit per test1 second
memory limit per test256 megabytes
inputstandard input
outputstandard output
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.

Examples
inputCopy
3 17 4
outputCopy
13

记得考虑特殊情况

#include <cstdio>//不难，但就是又坑点，这就是CF的妙处

using namespace std;

int main()
{
    int k,n,w;
    while(~scanf("%d%d%d",&k,&n,&w))//1e9应该不会爆int的吧，数据范围要牢记。
    {
        int sum = 0;
        for(int i = 1;i <= w;i++)
        {
            sum += k * i;
        }
        if(sum >= n)//我靠，要考虑不用借钱的情况。。。
            printf("%d
",sum - n);
        else printf("0
");
    }
    return 0;
}