var i,max,mtemp:integer;
    s,kttemp,ktmax:string;
begin
    readln(s);
    mtemp  := 1;
    max    := 0;
    i      := 1;
    kttemp := s[1];
    ktmax  := s[1];
    for i:=2 to length(s)+1 do 
    begin
        if s[i]=kttemp then 
            inc(mtemp)
        else
        begin
            if mtemp>=max then 
            begin
                ktmax := kttemp;
                max := mtemp;
            end;
            mtemp  := 1;
            kttemp := s[i];
        end;
    end;
    for i:=1 to max do write(ktmax);
    readln;
end.