#include<stdio.h>
#include<iostream.h>

/// -97
long v[25],n,i,poz=-1; char s;


int cauta()
{int i;
 for(i=0;i<poz;i++)
 if(v[i]>0) return poz=i;
 for(i=poz+1;i<25;i++)
 if(v[i]>0) return poz=i;
}


main()
{freopen("ordine.in","r",stdin);
 freopen("ordine.out","w",stdout);

 while(!feof(stdin)) {s=fgetc(stdin); v[s-97]++; n++;}

 for(i=0;i<n;i++)
 printf("%c",cauta()+97);

}