input: Write a function to calculate the Fibonacci sequence up to a given number 'n'. If 'n' is less than 0, print "Incorrect input". If 'n' is 0, return 0. If 'n' is 1, return 1. Otherwise, return the nth Fibonacci number.
  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(14) == 377
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