input: Write a function that returns the nth number in the Fibonacci sequence. If the input is less than 0, the function should print "Incorrect input" and return None. If the input is 0, the function should return 0. If the input is 1, the function should return 1. For any other positive integer input, the function should calculate the Fibonacci number and return it.
  def fibonacci(n): 
      a = 0
      b = 1
      if n < 0: 
          print("Incorrect input") 
          return None
      elif n == 0: 
          return a 
      elif n == 1: 
          return b 
      else: 
          for i in range(2, n+1): 
              c = a + b 
              a = b 
              b = c 
          return b 
  
  assert fibonacci(0) == 0
  assert fibonacci(1) == 1
  assert fibonacci(2) == 1
  assert fibonacci(3) == 2
  assert fibonacci(5) == 5
  assert fibonacci(9) == 34
  assert fibonacci(-1) == None
output: def fibonacci(n): 
      a = 0
      b = 1
      if n < 0: 
          print("Incorrect input") 
          return None
      elif n == 0: 
          return a 
      elif n == 1: 
          return b 
      else: 
          for i in range(2, n+1): 
              c = a + b 
              a = b 
              b = c 
          return b