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 num = in.nextInt();

        int sum = 2;
        for (int i = 1; i < num; i++) {
            sum += 2 + 3 * i;
        }
        System.out.println(sum);
    }
}