21st

Bottom to bottom Tabata squats.

A Tabata interval is 20 seconds "work" 10 seconds "rest" of the exercise for 8 intervals, which is 4 minutes.

Bottom to bottom means that you start the squats in the bottom (or squatted position) and squat during the 20 seconds, then rest in the squatted position without resting your hands on your legs or losing your good squatting position.

Your "score" is the least number of squats you are able to perform in any of the 8 intervals.