input: Write a function to compute the nth Fibonacci number. The function should return the Fibonacci number corresponding to the given input n. If the input is 0, return 0. If the input is 1, return 1. For any other value of n, return the sum of the Fibonacci numbers of n-1 and n-2.
  def fibonacci(n):
   if n == 0:
   return 0
   elif n == 1:
   return 1
   else:
   return fibonacci(n - 1) + fibonacci(n - 2)
   
  assert fibonacci(6)==8
  assert fibonacci(0)==0
  assert fibonacci(1)==1
  assert fibonacci(10)==55
  assert fibonacci(20)==6765
output: def fibonacci(n):
   if n == 0:
   return 0
   elif n == 1:
   return 1
   else:
   return fibonacci(n - 1) + fibonacci(n - 2)