input: Write a function to calculate the Fibonacci sequence up to the nth term. The function should return the nth Fibonacci number. If the input is less than 0, return "Incorrect input".
  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): 
              c = a + b 
              a = b 
              b = c 
          return b 
  
  assert fibonacci(9) == 21
  assert fibonacci(0) == 0
  assert fibonacci(1) == 1
  assert fibonacci(-1) == "Incorrect input"
  assert fibonacci(5) == 3
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