follow is my program:
var
 a:array[1..10000]of longint;
 z:array[1..10000]of integer;
 i,j,k,n:longint;
begin
 fillchar(a,sizeof(a),0);
 fillchar(z,sizeof(z),0);
 read(n);
 for i:=1 to n do begin read(a[i],j);z[i]:=i;end;
 for i:=1 to n-1 do
  for j:=i+1 to n do
   if a[z[i]]>a[z[j]] then
    begin
     k:=z[i];
     z[i]:=z[j];
     z[j]:=k;
    end;
 i:=0;
 while i<n do
  begin
   writeln(z[i+1],' ',z[i+2]);
   inc(i,2);
  end;
end.

Try this:
4
1 1
1 3
1 2
1 4
Your program says
1 2
3 4
but those roads intersect.
My first solution was wrong also, but I sort towns by polar angle and I can choose center randomly.
TO JUDGES: Can you add few tests that catch solutions, which sort towns only by X or by Y?

Your solution looks so strange!
Can you tell me why it works?

Ha!
Very funny!
I got it!
Oh, yeah!!!!!!!!!!!!!

Oh, you needn't explain it to me any more.
I have understood it!

bravoooo!!! Your idea is very good.

I think this is incorrect test. Because, No three towns lay on one line.