input: Write a function to calculate the nth number in the Fibonacci sequence. The function should return the nth number in the sequence. If the input is less than 0, the function should print "Incorrect input".
  def fibonacci(n):
      a = 0
      b = 1
      if n < 0:
          print("Incorrect input")
      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(9) == 34
  assert fibonacci(-1) == None
output: def fibonacci(n):
      a = 0
      b = 1
      if n < 0:
          print("Incorrect input")
      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