I did the same experiment. Got a sheet of paper and figured out the concept behind this problem within a minute but i took me quite some time to put it into code. Wrong logic / not perfectly concentrated / choosing the wrong way for a moment etc.

Am i a bad programmer now if i cant get a running program in less than 5 minutes ?

I would rather let people apply with code they are proud of like in every design job than solving litmus tests/puzzles