import java.util.*;
class Main{
        public static void main(String [] args){
        Scanner sc = new Scanner(System.in);
                int n = sc.nextInt();
        int s = sc.nextInt();
        List<String> res = new ArrayList();

        for(int i = 0; i <= n; i++){
                res.add(sc.nextLine());
        }

        Collections.sort(res);
        StringBuffer ret = new StringBuffer();
        for(String a : res)
          ret.append(a);

        System.out.println(ret.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