I'm developing a questions game. My goal is that the score for each correct answer will increase as the user answers more questions. Initially there are 15 points for each correct answer. Every 4 questions adds 2 points to the previous value. In terms of numerical series would be something like: 

    15 15 15 15 17 17 17 17 19 19 19 19 21 21 21 21.

I trying to find the formula for this numerical serie but I have not had success. If anyone sees in this numerical series a "challenge" and wants to help me find the formula I will be very grateful.