{ fp }

{* 
 * Problem  : 	SUBSTR
 * Contest  : 	UOI-2006 (Dnepropetrovsk)
 * Type     : 	Solution
 * Date     : 	April 1, 2006
 * Author   : 	Sergey Savchuk
 * Language :	Pascal
 * Compiler :	Free Pascal
 * Algorithm:	Fast solution (without explicit buffer specification), with optimized returning of the best string (usage of an array)
 *}


Var fi,fo: Text;
    s: string; ls: byte absolute s;
    a:array['a'..'z'] of Longint;
    c:array[0..10000000] of char;
    na,i,k,max,maxna: longint;
    ch: char;

Begin
     Assign(fi,'SUBSTR.DAT'); Reset(fi); na:=1;

     While not Eof(fi) do Begin Readln(fi,s);
      For i:=1 to ls do Begin inc(k); c[k]:=s[i] End
     End;

     For i:=1 to k do Begin
       if na<=a[c[i]] Then na:=a[c[i]]+1;
       if i-na+1>max Then Begin Max:=i-na+1; maxna:=na End;
       a[c[i]]:=i;
     End;

     Assign(fo,'SUBSTR.SOL'); ReWrite(fo);
     For i:=1 to max do Write(fo,c[maxna+i-1]); Writeln(fo);
     Close(fo)
End.