input: Write a function to generate the nth term of the Fibonacci sequence. The function should take an integer n as input and return the nth Fibonacci number. If the input is less than 0, the function should print "Incorrect input" and return None. The function should return the correct Fibonacci number for non-negative integer inputs.
  
  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(0) == 0
  assert fibonacci(1) == 1
  assert fibonacci(2) == 1
  assert fibonacci(9) == 34
  assert fibonacci(-1) == None
  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