import java.util.*;

class Main {
    public static void main(String[] args) {
        new Solver().run();
    }
}

class Solver {

    public void run() {
        Scanner sc = new Scanner(System.in);
        int N = sc.nextInt();
        int L = sc.nextInt();
        List<String> S = new ArrayList<String>();
        for (int i = 0 ; i < N; i++) {
            S.add(sc.next());
        }
        Collections.sort(S);

        StringBuilder ans = new StringBuilder();
        for (String s :S) {
            ans.append(s);
        }
        System.out.println(ans.toString());

        return;
    }

}
/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