input: Write a function that calculates the nth number in the Fibonacci sequence. The function should return the nth Fibonacci number. If the input is less than 0, it should print "Incorrect input". If the input is 0, it should return 0. If the input is 1, it should return 1. For other inputs, it should calculate the Fibonacci number as the sum of the previous two numbers in the sequence.
  
  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(9) == 34
  assert fibonacci(0) == 0
  assert fibonacci(1) == 1
  assert fibonacci(5) == 5
  assert fibonacci(10) == 55
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