The nth Fibonacci Number

The Fibonacci sequence starts with 0 and 1. The 0th Fibonacci number is 0, and the 1st is 1. From index 2 on, each number is the sum of the two numbers before it.

As a formula, $F_n = F_{n-1} + F_{n-2}$ for $n \ge 2$.

Listed up to $n = 17$, the sequence is:

$0,\ 1,\ 1,\ 2,\ 3,\ 5,\ 8,\ 13,\ 21,\ 34,\ 55,\ 89,\ 144,\ 233,\ 377,\ 610,\ 987,\ 1597$

Given $n$, write a program that computes the $n$th Fibonacci number.

The first line contains $n$. It is either 0 or a natural number less than or equal to 20.

Print the $n$th Fibonacci number on the first line.