import java.util.*;
 
class Main {
  
    public static void main(String[] args) {
      
        Scanner sc = new Scanner(System.in);
        int N = Integer.parseInt(sc.next());
        int L = Integer.parseInt(sc.next());
        String moji[] = new String[N];
      
      for(int i=0;i<N;i++){
        moji[i] = sc.next();
      }

      Arrays.sort(moji);
      for(int i=0;i<N;i++){
        System.out.print(moji[i]);
      }
      
    }
}
/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