import java.util.*;

public class Main {
    public static void main(String[] args) throws Exception {

        Scanner sc = new Scanner(System.in);
        int N = sc.nextInt();
        int L = sc.nextInt();
        String[] S = new String[N]; 
        
        //文字列を格納し、辞書順に並び替え
        for(int i = 0; i < N; i++)S[i] = sc.next();
        Arrays.sort(S);
        
        //可変長のクラスを利用した文字列結合
        StringBuilder bf = new StringBuilder();
        for(int i = 0; i < N; i++)bf.append(S[i]);
        
        String ans = bf.toString();
        System.out.println(ans);
    }
}

/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