const
 nmax=500;
type
 s=array[-1..nmax] of integer;
 s1=array[1..2] of integer;
var
 n:integer;
 a:array[1..nmax] of s;
 i,j,k,min,max:integer;
 M:array[1..nmax] of s1;
 b:array[1..nmax] of integer;
 procedure init;
  begin
   fillchar(a,sizeof(a),0);
   fillchar(b,sizeof(b),0);
   readln(n);
   for i:=1 to n do
    begin
    readln(m[i,1],m[i,2]);
     if m[i,1]>m[i,2] then  begin
            j:=m[i,1];
           m[i,1]:=m[i,2];
           m[i,2]:=j;  end;

    end;
  end;

 function online(x,y:integer):boolean;   {x in y}
  begin
  if ((m[y,1]<m[x,1]) and (m[x,2]<m[y,2])) and (m[x,1]<>m[x,2]) then
online:=true
                                 else online:=false;
  end;

 procedure check(k:integer);
  var i:integer;
  begin
   for i:=1 to n do
    if online(k,i) then
               begin
                a[i,k]:=1;
                a[k,i]:=1;
             if   a[k,-1]<=a[i,-1] then
                a[k,-1]:=a[i,-1]+1;
               end;

  end;

 function find:integer;
  var max:integer;
      i:integer;
  begin
   max:=1;
  for i:=2 to n do
   if a[max,-1]<a[i,-1] then max:=i;
    find:=max;
  end;

 procedure print(k:integer);
  var
   s:integer;
  begin
  s:=1;
  if a[k,-1]=0 then begin
                     writeln(k);
                     halt;
                    end;
   for i:=1 to n do
    if a[i,k]=1 then
     begin
      if a[i,-1]=a[k,-1]-1 then begin write(k,' '); print
(i);exit;end;

     end;

  end;

begin
 init;
 for i:=1 to n do check(i);
 writeln(a[find,-1]+1);
 i:=find;
 print(i);
end.

you could use qsort at first and compare two line