B - Soldier and Bananas

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 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

Input

3 17 4

Output

13
解题思路：等差数列求前w项和，如果够钱，则输出0，否则就输出需要借钱的数目，水过。
AC代码：

 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 }