1 pullup (or adaptation, based on your level of fitness)
3 full squats (getting your hips below your knees' standing level)
...as many times as you can, in 20 minutes. Mark noted in the comments that a very fit male should be able to do around 50 cycles of this (!!) in that time frame.
Me? 20 cycles after 16:10, at which point I stopped before I passed out underneath the pullup bar, leaving my 3.5 year old and 8 month old to fend for themselves on a public playground. I was sweating like an iced tea in the sun on the Fourth of July. My palms are slighly worse for wear - not blistered, but definitely victims of friction.
I have never - that I can recall - done a full body unassisted pullup in my life, even during my years of swimming in high school swim team. So - if Primal Blueprint Fitness ever gets me to a point of doing one Grade A genuine pullup, I might literally burst into tears of joy. But getting there requires both strength building and weight loss. For now, I'm doing Mark's suggested adaptation of "from one foot propped below, then pulled up as high as possible".
All this said, if you don't have a pullup bar at your disposal, check out one of your local playgrounds - odds are very good that even if there's no "official" pullup bar, there are enough high-up horizontal poles to serve the purpose. Go early in the morning for a vacated playground and lowest temps, but bring a towel in case you need to wipe the morning dew off the bars. Also carefully test the bar for sturdiness with respect to your body weight.