题解 | #统计每个月兔子的总数#

import java.util.Scanner;

public class Main {
    public static void main(String[] args) {
        Scanner in = new Scanner(System.in);
        int n = in.nextInt();
        int[] dp = new int[3];
        // 下标0,1,2表示出生1、2、3个月的兔子
        dp[0] = 1;
        for (int i = 1; i < n; i++) {
            dp[2] = dp[1] + dp[2];
            dp[1] = dp[0];
            dp[0] = dp[2];
        }
        System.out.println(dp[0] + dp[1] + dp[2]);
    }
}