546A.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>

int main(){

    int k = 0,n = 0,w = 0;
    int total = 0, ans = 0;
    std::cin>>k>>n>>w;

    total = k*((w*(w+1))/2);
    if(n>=total){
        std::cout<<0;
    }
    else{
        ans = total - n;
        std::cout<<ans;
    }

    return 0;
}