题目：古典问题：有一对兔子，从出生后第3个月起每个月都生一对兔子，小兔子长到第三个月

　　　后每个月又生一对兔子，假如兔子都不死，问每个月的兔子总数为多少？-Title: Classical: one pair of rabbits, one pair of rabbits are born from the month from 3 months after birth, a small rabbit gave birth to one pair of rabbits grow up to the third month after month, if the rabbit is not dead, asked Each month the total number of rabbits for how much?