//uses crt;
var s,t,kq:ansistring;
kt :array['A'..'Z'] of longint;
n,m:array[1..100000000] of longint;
i,j,max,richness,k:longint;
begin
        //clrscr;
    readln(s);
    for i:=1 to 2500 do kq:=kq+'Z';
    max:=0;
    for i:=1 to length(s) do
    begin
        fillchar(kt,sizeof(kt),0);
        richness:=0;
        for j:=i to length(s) do
                begin
                        if kt[s[j]]=0 then begin  inc(richness); inc(kt[s[j]]); end else
                        if kt[s[j]]=1 then begin  dec(richness); inc(kt[s[j]]); end;
                        if richness > max then max:= richness;
                end;
    end;
    k:=0;
    for i:=1 to length(s) do
    begin
        fillchar(kt,sizeof(kt),0);
        richness:=0;
        for j:=i to length(s) do
                begin
                        if kt[s[j]]=0 then begin  inc(richness); inc(kt[s[j]]); end else
                        if kt[s[j]]=1 then begin  dec(richness); inc(kt[s[j]]); end;
                        if richness = max then begin inc(k); m[k]:=i;   n[k]:=j; end;
                end;
    end;
    for i:=1 to k do
    begin
        t:='';
        for j:=m[i] to n[i] do t:=t+s[j];
        if t<kq then kq:=t;
    end;
    write(kq);
end.