A soldier wants to buy *w* bananas in the shop. He has to pay *k* dollars for the first banana, 2*k* 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?
The first line contains three positive integers *k*, *n*, *w* (1  ≤  *k*, *w*  ≤  1000, 0 ≤ *n* ≤ 10<sup class="upper-index">9</sup>), the cost of the first banana, initial number of dollars the soldier has and number of bananas he wants.
#include<iostream>
using namespace std;
#define ll long long

int main() {
    int t,k,w;
    long long n;
    int a;
    cin>>k>>n>>w;
    int sum = (w / 2) * (k + w * k);
    if (sum>n)
    {
      cout<<sum-n;
    }
   else{
   cout<<0;}
    
         
    }
