input: Write a function to calculate the Fibonacci sequence up to the nth term. If the input is less than 0, return an error message. If the input is 0, return 0. If the input is 1, return 1. Otherwise, return the nth term in the sequence.
  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(-1)=="Incorrect input"
  assert fibonacci(0)==0
  assert fibonacci(1)==1
  assert fibonacci(9)==34
  assert fibonacci(15)==610
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