program p1207;
uses math;
type
  realer=record
           rr:real;
           num:longint;
         end;
var
  i,j,k,n,m,x,y,x0,y0:longint;
  r:array[1..10000]of realer;
procedure qs(h,t:longint);
  var
    i,j:longint;
    k:real;
    o:realer;
  begin
    if h>=t
      then exit;
    i:=h;j:=t;k:=r[(i+j)div 2].rr;
    repeat
      while r[i].rr<k do
        inc(i);
      while r[j].rr>k do
        dec(j);
      if i<=j
        then begin
          o:=r[i];
          r[i]:=r[j];
          r[j]:=o;
          inc(i);
          dec(j);
        end;
    until i>j;
    qs(h,j);
    qs(i,t);
  end;
begin
  readln(n);
  readln(x0,y0);
  for i:=2 to n do
    begin
      readln(x,y);
      x:=x-x0;
      y:=y-y0;
      if x=0
        then
          if y>0
            then r[i-1].rr:=0.5*pi
            else r[i-1].rr:=-0.5*pi
        else r[i-1].rr:=arctan(y/x);
      r[i-1].num:=i;
    end;
  qs(1,n-1);
  writeln(1,' ',r[n div 2].num);
end.

Any hints or tests can be helpful! Thanks!

This test helped me to kill WA2

2
1 1
-1 -1

And don't forget about size of types!

4
-999999999 1000000000
-1000000000 999999999
1000000000 -1000000000
-1000000000 1000000000

out: 3 4