The Fibonacci sequence is the series of numbers where each number is the

sum of the two preceding numbers. The first Fibonacci number is $0,$ the second

Fibonnoci number is $1,$ the third Fibonnaci number is $1,$ the fourth Fibonacci

$2,$ the fifth Fibonacci number is $3,$ the sixth Fibonacci number is $5$ and so on$.$

Write a function called f ibonacci$($n$)$ that takes a positive integer n as an argument and returns the nth Fibonacci number. For example, F ibonacci$(9)$ should

return $21.$

Hint: Use a while loop.