Need advice...

Good evening... Need some help about how to correct my code to solve stair-steps:

    def checkio(numbers):
        if not numbers or len(numbers) == 1 and numbers[0] < 0:
            return 0
        else:
            if numbers[0] > 0:
                return numbers[0] + checkio(numbers[1:])
            else:
                return numbers[1] + checkio(numbers[2:])

I have a problem with last test case ((-10, -1, -1, -10)).