The first thing we need to do is handle all the edge cases. If n is less than 1, we simply have to return -1. If n is 1 or 2, that means we have to return the first or second value in the series.

Since we already know these are always 0 and 1, we can easily check for them at the start.

Now begins our while loop. A for loop would work just as well. Our iterator is the count variable, which starts from 3 because we already handled the first two values in the series.

In every iteration, we use the previous two terms in the sequence, which are second and fib_n. These become first and second in the next iteration.

In the end, fib_n will hold the final nth value in the Fibonacci sequence.