546A. Soldier and Bananas

import java.util.Scanner;


public class A546 {

    static void run(){
        Scanner in = new Scanner(System.in);
        int k =in.nextInt();
        int n = in.nextInt();
        int w = in.nextInt();
        int count = 0;
        count = (1+w)*w*k/2;
//        System.out.println(count);
        if(count>n){
            System.out.println(count-n);
        }else
            System.out.println(0);
    }
    public static void main(String[] args){
        run();
    }
}

def A546():
    k,n,w = map(int, raw_input().split())
    count = (1+w)*w*k*0.5
    if count > n :
        print int(count -n) 
    else:
        print 0 

if __name__==‘__main__‘:
    A546()