
import java.util.*;

/**
 * @author mix
 */
public class Main {

    public static void main(String[] args) {
        // TODO code application logic here
        Scanner s1 = new Scanner(System.in);
        int N = s1.nextInt();
        int L = s1.nextInt();
        String[] stringArray = new String[N];
        for (int i = 0; i < N; i++) {
            stringArray[i] = s1.next();
        }
        Arrays.sort(stringArray);
        for (String s : stringArray) {
            System.out.print(s);
        }
        System.out.println("");
    }

}

/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