import java.io.*;
import java.util.*;
public class Main {
	
	public static void main(String[] args) {
	
		MyScanner sc = new MyScanner();
		int n=sc.nextInt();
		int l=sc.nextInt();
		String s[]=new String[n];
		for(int i=0;i<n;i++) {
			s[i]=sc.next();
		}
		Arrays.sort(s);
		
		StringBuilder bf=new StringBuilder();
		for(int i=0;i<n;i++) {
			
			bf.append(s[i]);
		}
		String ans=bf.toString();
		System.out.println(ans);
	}
	
	public static PrintWriter out;

	public static class MyScanner {
		BufferedReader br;
		StringTokenizer st;

		public MyScanner() {
			br = new BufferedReader(new InputStreamReader(System.in));
		}

		String next() {
			while (st == null || !st.hasMoreElements()) {
				try {
					st = new StringTokenizer(br.readLine());
				} catch (IOException e) {
					e.printStackTrace();
				}
			}
			return st.nextToken();
		}

		int nextInt() {
			return Integer.parseInt(next());
		}

		long nextLong() {
			return Long.parseLong(next());
		}

		double nextDouble() {
			return Double.parseDouble(next());
		}

		String nextLine() {
			String str = "";
			try {
				str = br.readLine();
			} catch (IOException e) {
				e.printStackTrace();
			}
			return str;
		}

	}
	

}




/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