Problem 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·kdollars for the i-th banana).

He has n dollars. How many dollars does he have to borrow from his friend soldier to buy w

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

3 17 4

13
解題思路：等差數列求前w項和，如果夠錢，則輸出0，否則就輸出需要借錢的數目，水過。
AC代碼：

 1 #include<bits/stdc++.h>
 2 using namespace std;
 3 int main(){
 4     int k,n,w,r;
 5     cin>>k>>n>>w;
 6     r=((1+w)*w/2)*k;
 7     if(r<=n)cout<<‘0‘<<endl;
 8     else cout<<(r-n)<<endl;
 
 9     return 0;
10 }

B - Soldier and Bananas