we get this url: http://198.211.115.81:5001/
in there we can see a json list with some info about the users,
so we try diffrent things on the url until we find out that http://198.211.115.81:5001/user/1
(reminder: 1 is the id of the first user)
and in there we see the info of the first user(in json), then we try http://198.211.115.81:5001/user/2 and we see the info of the 
second user, so then we try 0 http://198.211.115.81:5001/user/2 and see nothing (weird),
finally we try -1  http://198.211.115.81:5001/user/-1
there we see the flag: KCTF{tHeRe_1s_n0_l1m1t}