input: Write a function to calculate the nth term of the Fibonacci sequence. If the input is less than 0, return an error message. For an input of 0, return 0 and for an input of 1, return 1. For all other inputs, return the nth term calculated by the formula.
  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 
  
  assert fibonacci(0) == 0
  assert fibonacci(1) == 1
  assert fibonacci(2) == 1
  assert fibonacci(3) == 2
  assert fibonacci(9) == 34
  assert fibonacci(-1) == "Incorrect input"
output: 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