Hj37 statistics of the total number of rabbits per month Fibonacci series

HJ37  Count the total number of rabbits every month     Fibonacci sequence

There is a kind of rabbit , From the day after birth 3 A rabbit has been born every month since last month , The little rabbit grows to the third month and gives birth to another rabbit every month .

Example ： Suppose a rabbit 3 Months was born , So it's the first 5 A rabbit will be born every month from the beginning of next month .

There was a rabbit in January , If the rabbits don't die , Ask No n What's the total number of rabbits in a month ？

Data range ： Input satisfaction  1 \le n \le 31 \1≤n≤31 

Input description ：

Enter a int An integer of type denotes the th n Months

Output description ：

Output the corresponding total number of Rabbits

ip = input()
def df(n):
    if n < 3: return 1
    return df(n-1) + df(n-2)
print(df(ip))

This question is very interesting ：

The number of rabbits per month = The number of rabbits last month （ Because rabbits don't die , So last month's rabbit , The current month is still alive ） + The new born rabbit in the current month （ The first born rabbit of the current month , The number of rabbits in the first two months , Because the rabbits in the first two months are adult rabbits , All have fertility ） = df(n-1) +df(n-2)

A typical Fibonacci sequence