题解 | #等差数列#

import java.util.*;


public class Main {
    public static void main(String[] args) {
        Scanner fzhinput = new Scanner(System.in);
        int num = fzhinput.nextInt();
        int sum=0;
        for(int i=0;i<num;i++){
            sum+=2+3*i;
        }
        System.out.println(sum);
    }
}