input: Write a function to find the nth Fibonacci number. If the input number is less than 0, print "Incorrect input" and return None. Otherwise, return the nth Fibonacci number.
  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(2)==1
  assert Fibonacci(5)==5
  assert Fibonacci(10)==55
output: def Fibonacci(n): 
      if n<0: 
          print("Incorrect input") 
          return None
      elif n==0: 
          return 0
      elif n==1: 
          return 1
      else: 
          return Fibonacci(n-1)+Fibonacci(n-2)