老鼠繁殖问题

开始有两只老鼠，每个月繁殖一对幼鼠，幼鼠三个月长大，即可繁殖，
问二十四个月后共有多少只老鼠（不考虑老鼠死亡、雌雄问题）？
package com.lovo.homework;

public class TestMouse {

/**
* @param args
*/
public static void main(String[] args) {
// TODO Auto-generated method stub

int old = 2,first = 0, second = 0, third = 0;

for(int i = 0; i < 24; i++){
old = old + third;
third = second;
second = first;
first = old;
}

System.out.println(old + first + second + third);
}

}