What should be in the brackets 0, - 1, 3, - 7, (), - 31, 63, - 127?

What should be in the brackets 0, - 1, 3, - 7, (), - 31, 63, - 127?

fifteen
The absolute value of the next digit = 2 * the absolute value of the previous digit + 1, then add a negative sign before the odd digit, and do not add a negative sign before the even digit