# include <stdio.h>
# include <string.h>
int i,ok,fr[300],x;
char ch,c,c2;
int main ()
{
freopen ("ordine.in","r",stdin);
freopen ("ordine.out","w",stdout);
while (scanf ("%c",&ch)!=EOF)
{
x=ch;
fr[x-97]++;
}

while (ok==0)
{
ok=1;
i=0;
while (i<26)
{
c2=i+97;
if (fr[i]!=0 && c!=c2)
{
ok=0;
fr[i]--;
c=i+97;
printf ("%c",c);
break;
}
i++;
}
}
return 0;
}