input: Write a function to calculate the nth number in the Fibonacci sequence. The function should return the Fibonacci number at the given position n. 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, the function should calculate and return the corresponding Fibonacci number.
  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(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") 
          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