3, -6, 12, 4, 20, what is the next number in sequence

Answer 1

I can only see one pattern:

down 9

up 2 times 9 = 18

down 8

up 2 times 8 =16

If that's it, then the sequence will be:

3, -6, 12, 4, 20, 13, 27, 21, 33, 28, 38, 34, 42, 39, 45, 43, 47, 46, 48,