```
"""
rabbits(n::Integer, k::Integer)
Problem: https://rosalind.info/problems/fib/
Given: Two numbers:
- n => number of months
- k => number of rabbits litter per each pair
Returns: Number of rabbits after n months
Contributed by:- [Nikola Mircic](https://github.com/Nikola-Mircic)
"""
function rabbits(n::Integer, k::Integer)
# For calculating the number of rabbit pairs, we will only need the last two months
# and instead of the array with n elements, we will only use two variables
a = 1 # First generation
b = 1 # Second generation
# Calculate the number of rabbits for each generation from 3rd to nth
for i in 3:n
# Every rabbit pair older than month will give 3 new pairs
# so the number of rabbits in k-th month is a[k] = a[k-1] + a[k-2] * 3
t = b
b = b + 3 * a
a = t
end
return b
end
```