type
 tfile = file of integer;
var
 t,p: integer;
 a,b,c,d: tfile;
 f,fsort: boolean;

{--Sorting: realizing mearge sort. return status end of sorting--}
function sort(var a,b,c,d: tfile): boolean;
var
 fi,fj,i,j,ta,tb,pa,pb: integer;
begin
 i:=2;j:=2;
 reset(a); reset(b); rewrite(c); rewrite(d);
 while (i<=filesize(a)) and (j<=filesize(b)) do begin
   seek(a,i-2); seek(b,j-2); read(a,pa); read(b,pb);
   seek(a,i-1); seek(b,j-1); read(a,ta); read(b,tb);
   while (ta>=pa) and (tb>=pb) and (i<=filesize(a)) and (j<=filesize(b)) do begin
     seek(a,i-2); seek(b,j-2); read(a,pa); read(b,pb);
     seek(a,i-1); seek(b,j-1); read(a,ta); read(b,tb);
     if f then if ta>tb then begin write(c,tb); inc(j); end
                        else begin write(c,ta); inc(i); end
          else if ta>tb then begin write(d,tb); inc(j); end
                        else begin write(d,ta); inc(i); end;
   end;
   seek(a,i-2); seek(b,j-2); read(a,pa); read(b,pb);
   seek(a,i-1); seek(b,j-1); read(a,ta); read(b,tb);
   while ((ta>=pa) or (tb>=pb)) and (i<=filesize(a)) and (j<=filesize(b)) do begin
     seek(a,i-2); seek(b,j-2); read(a,pa); read(b,pb);
     seek(a,i-1); seek(b,j-1); read(a,ta); read(b,tb);
     if f then begin
       if ta>=pa then begin write(c,ta); inc(i); end;
       if tb>=pb then begin write(c,tb); inc(j); end;
     end else begin
       if ta>=pa then begin write(d,ta); inc(i); end;
       if tb>=pb then begin write(d,tb); inc(j); end;
     end;
   end;
 end;
 close(a); close(b); close(c); close(d);
end;

begin
 {--Initialization: assigning and make file b.txt--}
 assign(a,'a.txt'); assign(b,'b.txt'); assign(c,'c.txt'); assign(d,'d.txt');
 rewrite(b); close(b);

 {--First read: data moved from a to c and d--}
 assign(input,'a.txt');  reset(input);
 assign(c,'c.txt');  reset(c);
 assign(d,'d.txt');  reset(d);
 f:=true;
 read(t);
 while not eof do begin
   p:=t;
   read(t);
   if f then write(c,p) else write(d,p);
   if p>t then f:=not f;
 end;
 if p>t then f:=not f;
 if f then write(c,t) else write(d,t);
 close(input); close(c); close(d);

 {--Haupt sorting: using procedure sort with either params--}
 f:=true; fsort:=false;
 while not fsort do begin
   if f then fsort:=sort(c,d,a,b) else fsort:=sort(a,b,c,d);
   f:=not f;
 end;

 {--Preperer to out: moving result file in c.txt--}
 reset(a); rewrite(c);
 if not f then
   while not eof(a) do begin
     read(a,t);
     write(c,t);
   end;
 close(a); close(c);  

 {--Output: export all digits data from c.txt in a.txt--}
 assign(output,'a.txt');
 rewrite(output); reset(c);
 while not eof(c) do begin
   read(c,t);
   write(t);
 end;
 close(output); close(c);
end.