input: Write a function to find the nth Fibonacci number. If the input is less than 0, print "Incorrect input". If the input is 0, return 0. If the input is 1, return 1. For any other positive integer n, return the sum of the (n-1)th and (n-2)th Fibonacci numbers.
  def Fibonacci(n): 
      if n<0: 
          print("Incorrect input") 
      elif 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(5)==5
  assert Fibonacci(10)==55
  assert Fibonacci(20)==6765
output: def Fibonacci(n): 
      if n<0: 
          print("Incorrect input") 
      elif n==0: 
          return 0
      elif n==1: 
          return 1
      else: 
          return Fibonacci(n-1)+Fibonacci(n-2)