2008 Hyundai Elantra cranks over but does not start on firs

Engine Mechanical problem
2008 Hyundai Elantra 4 cyl Two Wheel Drive 38800 miles

I put gas one evening and the next morning when going to start the car it did not start on the first crank. I cranked it again and it started right up. If the car sits for more than about 20 or 30 mins. It will not start on first crank. If you turn it off and try starting again within a couple of minutes of turning the car off, it starts right up on first crank. What can cause this? The only thing I did was put gas before this started. Before that no problems. The car runs fine. No hesitation or stalling. The check engine light hasn't came on at all either.