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 }