input: Write a function to find the nth Fibonacci number. Return the nth Fibonacci number if n is positive, return 0 if n is 0, and print "Incorrect input" if n is negative.
  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(3)==2
  assert Fibonacci(5)==5
  assert Fibonacci(0)==0
  assert Fibonacci(10)==55
  assert Fibonacci(-1)=="Incorrect input"
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)