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);
        int n = sc.nextInt();
        int a1 = 2;
        int d = 3;
        //An = a1+(n-1)d = Am+(n-m)d  
        //Sn = n*a1+n(n-1)d/2 = n(a1+an)/2
        int sn = n*a1 + n*(n-1)*d/2;
        System.out.println(sn);
    }
}