import java.util.ArrayList;
import java.util.Collections;
import java.util.Comparator;
import java.util.HashMap;
import java.util.Iterator;
import java.util.List;
import java.util.Map.Entry;
import java.util.Scanner;

public class StringChat {

	public static void main(String[] args) {
		
		Scanner sc = new Scanner(System.in);
		String s = sc.nextLine();
		sc.close();
		HashMap<Character, Integer> hashmap = new HashMap<Character, Integer>();
		for (int i = 0; i < s.length(); i++) {
			if (hashmap.containsKey(s.charAt(i))) {
				hashmap.put(s.charAt(i), hashmap.get(s.charAt(i)) + 1);
			} else {
				hashmap.put(s.charAt(i), 1);
			}
		}
		Iterator<?> iter = hashmap.entrySet().iterator();
		while (iter.hasNext()) {
			@SuppressWarnings("unchecked")
			Entry<Character, Integer> next = (Entry<Character, Integer>)iter.next();
			Entry<Character, Integer> entry = next;
			Character c = (Character) entry.getKey();
			Integer n = (Integer) entry.getValue();
			System.out.println(c + "  :  " + n);
		}
		List<Entry<Character, Integer>> list = new ArrayList<Entry<Character, Integer>>(hashmap.entrySet());
		Collections.sort(list, new Comparator<Entry<Character, Integer>>() {
			@Override
			public int compare(Entry<Character, Integer> o1, Entry<Character, Integer> o2) {
				return o1.getValue() - o2.getValue();
			}
		});
		StringBuilder strBuilder = new StringBuilder();
		for (int i = 0; i < list.size(); i++) 
			for (int j = 0; j < list.get(i).getValue(); j++)
				strBuilder.append(list.get(i).getKey());
		System.out.println(strBuilder.toString());
	}
	
}