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 in = new Scanner(System.in);
        int number = in.nextInt();
        int sum = 0;
        if(number == 1) {
            sum = 2;
        } else {
            sum += 2;
            for (int i = 1; i < number; i++) {
                sum += (3 * i + 2);
            }
        }
        System.out.println(sum);
    }
}