Problem
假設兔子需一個月性成熟，，性成熟後每對兔子每個月必繁殖 k 對子代，且不因任何因素死亡，則求 n 月後的兔子對數。
Given: Positive integers n≤40 and k≤5.
Return: The total number of rabbit pairs that will be present after n months, if we begin with 1 pair and in each generation, every pair of reproduction-age rabbits produces a litter of k rabbit pairs (instead of only 1 pair).
