Fibonacci Numbers

http://rosalind.info/problems/fibo/

The Fibonacci numbers 0,1,1,2,3,5,8,13,21,34,… are generated by the following simple rule
Fn=⎧⎩⎨⎪⎪Fn−1+Fn−2,1,0,n>1,n=1,n=0.

Given: A positive integer n≤25.

Return: The value of Fn.