If any triangle's sides are a,b, and c, then



So





We can ignore the third one, since it is implied by the middle one:



That is the same as:



So n can be any of these 15 values (in order) 

6, 7, 8, 9, 10, 11, 12, 13, 14, 15, 16, 17, 18, 19, 20

The median of an odd number of numbers is the one right smack
in the middle when they are arranged from smallest to largest,
or vice-versa.  That is the number I have colored red, which is 13.

Edwin