

import java.io.*;
import java.util.*;

class Main {
    public static void main(String[] args) throws IOException {
		
		// ** For submission. 
		Scanner sc = new Scanner(System.in);
		
		// Write your code here. 
		
        int n = sc.nextInt();
        int L = sc.nextInt();
        
        String[] strs = new String[n];
        for (int i = 0; i < n; i++) {
        	strs[i] = sc.next();
		}
        
        Arrays.sort(strs);
        
        StringBuilder sb = new StringBuilder();
        for (int i = 0; i < strs.length; i++) {
			sb.append(strs[i]);
		}
        
        System.out.println(sb.toString());
        
    }
}






































/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