题解 | #直接把兔子分类后硬算#

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);
        // 注意 hasNext 和 hasNextLine 的区别
        while (in.hasNextInt()) { // 注意 while 处理多个 case
            int a = in.nextInt();
            int r1 = 1;
            int r2 = 0;
            int r3 = 0;
            for (int i = 1; i < a; i++) {
                int temp = 0;
                temp = r2;
                r2 = r1;
                r3 = temp + r3;
                r1 = r3;       
            }
            int sum = r1 + r2 + r3;
System.out.println(sum);
        }
    }
}