Kata-cows.md

Consider having a cow that gives a child every year from her fourth year of life on and all her subsequent children do the same.

After n years how many cows will you have?

count_cows(0)  # should equal 1
count_cows(1)  # should equal 1
count_cows(3)  # should equal 2
count_cows(4)  # should equal 3
count_cows(10)  # should equal 28

Return None if n is not an integer.

Note: Assume all the cows are alive after n years.