input: Write a function to calculate the Fibonacci numbers up to a given number. The function should return the nth Fibonacci number.
  def Fibonacci(n):
      if n==0:
          return 0
      elif n==1:
          return 1
      else:
          return Fibonacci(n-1) + Fibonacci(n-2)
  
  assert Fibonacci(0)==0
  assert Fibonacci(1)==1
  assert Fibonacci(2)==1
  assert Fibonacci(3)==2
  assert Fibonacci(4)==3
  assert Fibonacci(5)==5
output: def Fibonacci(n):
      if n==0:
          return 0
      elif n==1:
          return 1
      else:
          return Fibonacci(n-1) + Fibonacci(n-2)