/*
* @Author Silviase(@silviasetitech)
* For ProCon
*/

import java.util.*;
import java.lang.*;
import java.math.*;

class Main{
    public static void main(String[] args){
        Scanner sc = new Scanner(System.in);
        
        int n = sc.nextInt();
        int len = sc.nextInt();
        List l = new ArrayList<String>();
        for (int i = 0; i < n; i++) {
            String tmp = sc.next();
            l.add(tmp);
        }
        Collections.sort(l);
        for (int i = 0; i < l.size(); i++) {
            System.out.print(l.get(i));
            if (i == l.size()-1) {
                System.out.println("");
            }
        }

        sc.close();
    }

}




/tmp/hf-datasets-cache/medium/datasets/42704881117311-config-parquet-and-info-qiankunmu-Project_CodeNet-896c9d36/hub/datasets--qiankunmu--Project_CodeNet_Python800_and_Java250/snapshots/c899fde44a77ba1cc14973ed3de064e4ff3aadc7/Project_CodeNet_Java250.tar.gz