import java.util.*;
public class Main {
    public static void main(String [] args){
        Scanner input = new Scanner(System.in);
        
        int N = input.nextInt();
        int L = input.nextInt();
        
        String a[] = new String[N];
        String s;
        for(int i = 0; i <N; i++){
            s = input.next();
            if(s.length()==L)
                a[i]=s;
        }
        Arrays.sort(a);
        for(int i = 0; i <a.length; i++)
             System.out.print(a[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