#include<stdio.h>
#include<iostream.h>
#include<string.h>
#define LMAX 100000

/// -97
long v[26],n,i,poz=-1; char sir[LMAX];


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


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

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

 gets(sir);
 for(i=0;i<strlen(sir);i++) v[sir[i]-97]++;

 for(i=1;i<=strlen(sir);i++)
 printf("%c",cauta()+97);    cout<<endl;


}