MetaPost: My code with buildcycle doesn't have the correct output and nothing changes

Instead of

kujund:=buildcycle(m, n);

try

kujund:=buildcycle(reverse m, n);

or

kujund:=buildcycle(m, reverse n);

Either way, it works for me:

By the way, the buildcycle macro can be tricky if used with only two paths. See the Metapost documentation p. 30-32 for more details about how it works. Given this way of working, it seems that by asking

buildcycle(m,n)

it leads Metapost to build a cycle between either only two points (I guess (0,-4) and the intersection of the two parabolas) or even only one point (I suppose (0,-4)) hence the absence of apparent result. Whereas

buildcycle(m, reverse n)

leads it to build the requested cycle between the expected intersection points.