题解 | #等差数列#

import java.util.Scanner;

// 注意类名必须为 Main, 不要有任何 package xxx 信息
public class Main {
    public static void main(String[] args) {
        Scanner sc = new Scanner(System.in);
        while (sc.hasNext()){
            int n = sc.nextInt();
            int sum = 0;
            for (int i = 2; i < 2+(n*3); i=i+3) {
                sum +=i;
            }
            System.out.println(sum);
        }
    }
}