题解 | #牛牛学数列6#

import java.util.Scanner;

public class Main{
    public static void main(String[] args){
        Scanner scan = new Scanner(System.in);
        int n = scan.nextInt();
        System.out.println(fun(n));
    }    
    public static int fun(int n){
        if(n == 1){
            return 0;
        }else if(n == 2 || n == 3){
            return 1;
        }
        return fun(n - 3) + 2 * fun(n - 2) + fun(n - 1);
    }
}