java

import java.util.*;
public class Main{
    public static void main(String[] arg){
        Scanner s=new Scanner(System.in);
        while(s.hasNextLine()){
            String ss=s.nextLine();
            int[] arr=new int[36];
            for(int i=0;i<ss.length();i++){
                if(ss.charAt(i)>=48&&ss.charAt(i)<=57){
                    arr[ss.charAt(i)-'0']=arr[ss.charAt(i)-'0']+1;
                }else{
                    arr[ss.charAt(i)-'a'+10]=arr[ss.charAt(i)-'a'+10]+1;
                }
            }

            int[] indexArr=new int[arr.length];
            for(int i=0;i<indexArr.length;i++){
                indexArr[i]=i;
            }
            int temp=0; int index=0;
            //排序--冒泡
            for (int i = 0; i < arr.length; i++) {
               for (int j = 0; j < arr.length - i - 1; j++) {
                    if (arr[j] < arr[j + 1]) {
                        temp = arr[j];
                        arr[j] = arr[j + 1];
                        arr[j + 1] = temp;

                        index = indexArr[j];
                        indexArr[j] = indexArr[j + 1];
                        indexArr[j + 1] = index;
                    }
                }
             }
            for(int i=0;i<arr.length;i++){
                if(arr[i]!=0){
                    System.out.print(indexArr[i]<10?(char)(indexArr[i]+'0'):(char)(indexArr[i]-10+'a'));
                }
            }
            System.out.println();
    }
  }
}