import java.util.*;

public class Main{
  public static void main(String[] args){
    Scanner sc=new Scanner(System.in);
    final int N=sc.nextInt();
    final int L=sc.nextInt();
    String[] s=new String[N];
    for(int i=0;i<N;i++){
      s[i]=sc.next();
    }
    Arrays.sort(s);
    for(int i=1;i<N;i++){
      s[0]+=s[i];
    }
    System.out.println(s[0]);
  }
}
/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