input: 
  def fibonacci(n): 
      a = 0
      b = 1
      if n < 0: 
          return "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 
  
  # Test cases
  assert fibonacci(0) == 0
  assert fibonacci(1) == 1
  assert fibonacci(9) == 34
  assert fibonacci(-1) == "Incorrect input"
  
output:     if n < 0: 
          return "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