#include <cstdio>

using namespace std;
int i,nr,u,a[35];
char c;

void ord(int a[35],int &u)
{
  int i;
  for(i=0;i<=25;++i)
  if(a[i]>0 && u!=i)
  {
    --a[i];
    u=i;
    break;
  }
  printf("%c",(char)i+97);
}

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

    while(0==0)
    {
       scanf("%c",&c);
       if(c=='\n') break;
       a[c-97]++;
       nr++;
    }

    u=100;
    while(nr>0)
    {
      nr--;
      ord(a,u);
    }

    printf("\n");




    return 0;
}