题解 | #等差数列#

import java.util.Scanner;

public class Main {
    public static void main(String[] args) {
        Scanner in = new Scanner(System.in);
        while (in.hasNextInt()) {
            int n = in.nextInt();
            int sum = 2;
            if(n == 1){
                System.out.println(2);
                return;
            }
            int preNum = 2;// 上一个等差数
            for(int i = 1;i<n;i++){
                sum = sum + preNum + 3;
                preNum += 3;
            }
            System.out.println(sum);
        }
    }
}