I have WA 10. I think I can't find coordinates center of circle very well. I think I have presision error. because in my solution I use numbers which maybe become larger than maxlongint;

if we have points. x1,y1 and x2,y2 and they are located at circle. how we can find coordinates cenrte of circle ?

One of these: (x1 + x2)/2, (y1 + y2)/2, isn't it :)

yes of course. but I missed one thing. radius should be fixed. we have points at circle x1,y1,x2,y2 and r. find the centre point of cirle which radius is r. :) how can I do it?

For example, you can consider isosceles triangle (two original points and center). You can also solve two equations: dist(p, p1) = r and dist(p, p2) = r in coordinats (but I think first way is simpler).