Welcome, stranger. I am the Builder, the one who conceived and built this Room, and you are its prisoner now. Do not rush into looking for an explanation of how you ended up in my room, rather try to solve my riddles to get out alive.
You will have to use your intellect and a pinch of imagination to achieve the final enigma and exit my Room
There's an old and shabby desk in front of you. There are numbers written on some parts of it, while one is missing, find out what it is and one of its drawers will unlock.
Good luck!

13 - 15 - 18 - 22 - 27 - ??