{ fp }

{* 
 * Problem  : 	SUBSTR
 * Contest  : 	UOI-2006 (Dnepropetrovsk)
 * Type     : 	Solution
 * Date     : 	April 1, 2006
 * Author   : 	Sergey Savchuk
 * Language :	Pascal
 * Compiler :	Free Pascal
 * Algorithm:	Slow solution with read optimization
 *}


Var fi,fo: Text;
    a:array['a'..'z'] of Longint;
    buf:array[0..10000] of char;
    i: longint;
    ch: char;
    S:String; ls: byte absolute s;
    M:String; lm: byte absolute m;

Begin
     Assign(fi,'SUBSTR.DAT'); Reset(fi); SettextBuf(fi,Buf,10000);

     While not Eof(fi) do Begin inc(i);
      Read(fi,ch); if ch<'a' Then continue;
      if a[ch]<>0 Then Delete(S,1,pos(ch,S));
       a[ch]:=i; S:=S+ch; if ls>lm Then M:=S
     End;

     Assign(fo,'SUBSTR.SOL'); ReWrite(fo); Writeln(fo,M); Close(fo)
End.