import java.util.*;

public class Main{
  public static void main(String[] args){
    Scanner sc = new Scanner(System.in);
    int N = sc.nextInt();
    int L = sc.nextInt();
    List<String> a = new ArrayList<>();
    // String a[] = new String[N];
    for(int i=0; i<N ;i++){
        a.add(sc.next());
        // a[i] = sc.next();
    }
    Collections.sort(a);
    String sum = "";
    for(String s : a){
        sum = sum.concat(s);
    }
        System.out.println(sum); 

    }    
}
/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