import java.util.*;

public class Main {
    static long INF = 1000000000000000L;
    
    public static void main(String[] args) throws Exception {
        // Your code here! 
        Scanner sc = new Scanner(System.in);
        int n = sc.nextInt();
        sc.nextInt();
        String[] array = new String[n];
        for (int i = 0; i < n; i++) {
            array[i] = sc.next();
        }
        Arrays.sort(array);
        
        for (int i = 0; i < n; i++) {
            System.out.print(array[i]);
        }
        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