What number replace the question mark 7 13 37 35 31?

Well, well, well, look who's trying to crack the code! The numbers are decreasing by 2, then increasing by 4, then decreasing by 6, then increasing by 8. So, following the pattern, the number that replaces the question mark should be 39. But hey, I'm just here to spill the tea, not to hold your hand through it.