Recursion in Python

In the very first year, there are $120$ frogs in the marsh. Once a year, $50$ frogs move to a nearby pond, while the remaining frogs breed so that their number doubles. Thus, for year $k$, you can calculate the number of frogs in the marsh based on the formula:

$F_k = 2(F_{k-1} - 50)$

Write a function number_of_frogs that returns this number for a given year.

Just implement the function, you don't have to handle input or print anything.