import java.util.*;
import java.util.Collections;
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 A[] = new String [N];
        for(int i = 0;i < N;i++){
            A[i] = sc.next();
        }
        Arrays.sort(A);
        StringBuilder B = new StringBuilder();
        for(int i = 0;i < A.length;i++){
            B.append(A[i]);
        }
        System.out.println(B);
    }
}

/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