input: Write a function to calculate the nth Fibonacci number. The function should return 0 for n = 0 and 1 for n = 1. For any other value of n, it should return the sum of the two preceding Fibonacci numbers. If the input value is less than 0, the function should return -1.
  assert fibonacci(6) == 8
  assert fibonacci(0) == 0
  assert fibonacci(1) == 1
  assert fibonacci(10) == 55
  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 _ in range(2, n + 1):
              a, b = b, a + b
          return b