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 n = 1;
        // 注意 hasNext 和 hasNextLine 的区别
        while (in.hasNextInt()) { // 注意 while 处理多个 case
            n = in.nextInt();
        }

        int sum = sum(n);
        System.out.print(sum);
    }

    private static int sum(int n){
        
        int sum =0;
        int a0 = 2;
        int an = 0;

        for(int i=0;i<n;i++){
            an = a0 + 3*i;
            sum += an; 
        }
        return sum;
    }
}