A - Soldier and Bananas

Time Limit:1000MS     Memory Limit:262144KB     64bit IO Format:%I64d & %I64u

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

题意：

士兵去买香蕉，买的第一根香蕉的价格为k，第i根的价格为i*k。给定士兵带的钱和需要买的香蕉数目，求士兵要借多少钱，如果不用借钱，则输出0.（由于没有判断输出0的条件WA了一发..冤枉啊！！！！！）

虽然是一道水题，但还是发出来劝解各位小伙伴和自己一定要认真读题，千万不要冤WA。

还是附AC代码：

 #include<iostream>
 using namespace std;

 int a[];

 int main(){
      int k,n,w,sum=;
      cin>>k>>n>>w;
     for(int i=;i<=w;i++){
         a[i]=i*k;
         sum+=a[i];
     }
     if(sum-n>)//！！！！！！
     cout<<sum-n<<endl;
     else
     cout<<""<<endl;
     return ;
 }