This is my programme which I think it's OK,but got WA in Test 7.
I'm glad to thank you if everyone can give some testdatas.

program ural1103;

{$APPTYPE CONSOLE}

const
        maxn=5000;

type
        point=record
                x,y     :extended;
        end;

var
        po      :array[1..maxn]of point;
        n       :integer;

procedure swap(a,b:integer);
var
        t       :point;
begin
t:=po[a];po[a]:=po[b];po[b]:=t;
end;

function min(a,b:integer):boolean;
begin
min:=(po[b].y-po[a].y>1e-10)or
        ((abs(po[b].y-po[a].y)<1e-10)and(po[b].x-po[a].x>1e-10));
end;

function dis(i,j:point):extended;
begin
dis:=sqrt((i.x-j.x)*(i.x-j.x)+(i.y-j.y)*(i.y-j.y));
end;

function cos_(a:point):extended;
begin
cos_:=(dis(a,po[1])*dis(a,po[1])+dis(a,po[2])*dis(a,po[2])
        -dis(po[1],po[2])*dis(po[1],po[2]))/(dis(a,po[1])*dis(a,po[2]));
end;

procedure qsort(p,q:integer);
var
        i,j     :integer;
        x       :point;
begin
if p>=q then exit;
i:=p-1;
j:=q+1;
x:=po[(i+j)div 2];
while(i<j)do
begin
        repeat inc(i);until (cos_(x)-cos_(po[i])>-1e-10);
        repeat dec(j);until (cos_(po[j])-cos_(x)>-1e-10);
        if i<j then swap(i,j);
end;
qsort(p,j);
qsort(j+1,q);
end;

function max(a,b:integer):boolean;
begin
max:=((po[a].x-po[1].x)*(po[b].y-po[1].y)
        -(po[b].x-po[1].x)*(po[a].y-po[1].y))>1e-10;
end;

procedure init;
var
        i       :integer;
begin
read(n);
for i:=1 to n do
        read(po[i].x,po[i].y);
for i:=2 to n do
        if min(i,1) then swap(1,i);
for i:=3 to n do
        if max(i,2) then swap(2,i);
qsort(3,n);
writeln(po[1].x:0:0,' ',po[1].y:0:0);
writeln(po[2].x:0:0,' ',po[2].y:0:0);
writeln(po[(n+3)div 2].x:0:0,' ',po[(n+3)div 2].y:0:0);
end;

begin
  { TODO -oUser -cConsole Main : Insert code here }
  init;
  readln;
//  readln;
end.

Edited by author 04.06.2004 17:55