Challenge: Fibonacci Number

Problem

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Fibonacci Number Problem

Compute the $n$-th Fibonacci number.

Input: An integer $n$.

Output: The $n$-th Fibonacci number.

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Fibonacci numbers are defined recursively:

$F_n = \begin{cases}n & \mathrm{if}\:n\:\mathrm{is}\:0\:\mathrm{or}\:1 \\F_{n-2} + F_{n-1} & \mathrm{if}\:n\geq2\end{cases}$ ...