SoldierAndBananas.cpp

/* 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. */

#include <iostream>

using namespace std;

int main()
{
    int k, w, n, e, b;
    cin >> k >> w >> n;
    for(int i = 1; i <= n; i++){
        b = k * i;
        e = e + b;
    }

    if(e > w){
       cout << e - w;
    }else{
        cout << 0;
    }

    return 0;
}