program p1;
var f,g:text;
    fr:array['a'..'z'] of longint;
    i,n:longint;
    c,prec:char;

function urmator(x:char):char;
var j:char;
begin
    for j:='a' to 'z' do
    if (fr[j]>0)and(j<>x) then break;
    fr[j]:=fr[j]-1;
    urmator:=j;
end;

begin
    assign(f,'ordine.in');reset(f);
    assign(g,'ordine.out');rewrite(g);
    n:=0;
    while not eoln(f) do
    begin
        read(f,c);
        n:=n+1;
        fr[c]:=fr[c]+1;
    end;
    prec:='*';
    i:=1;
    while i<=n do
    begin
        c:=urmator(prec);
        write(g,c);
        i:=i+1;
        prec:=c;
    end;
    writeln(g);
    close(f);
    close(g);
end.