import java.util.*;
public class Main { 
    public static void main(String[] args) {
        Scanner sc = new Scanner(System.in);
        int n = sc.nextInt();
        int len = sc.nextInt();
        String[] strs = new String[n];
        for(int i = 0; i < n; i++)
            strs[i] = sc.next();

        Arrays.sort(strs);
        StringBuilder sb = new StringBuilder();
        for(int i = 0; i < n; i++)
            sb.append(strs[i]);
        System.out.println(sb.toString());
    }
}
/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