input: Write a function to return the nth Fibonacci number. The Fibonacci sequence starts with 0 and 1, and each subsequent number is the sum of the previous two numbers. The function should return -1 if the input number is negative.
  def fibonacci(n):
      if n < 0:
          return -1
      elif n == 0:
          return 0
      elif n == 1:
          return 1
      else:
          a, b = 0, 1
          for i in range(2, n + 1):
              a, b = b, a + b
          return b
  
  assert fibonacci(5)==5
  assert fibonacci(10)==55
  assert fibonacci(0)==0
  assert fibonacci(1)==1
  assert fibonacci(-1)==-1
output: def fibonacci(n):
      if n < 0:
          return -1
      elif n == 0:
          return 0
      elif n == 1:
          return 1
      else:
          a, b = 0, 1
          for i in range(2, n + 1):
              a, b = b, a + b
          return b