import java.util.Arrays;
import java.util.Scanner;

public class Main {

	public static void main(String[] args) {
		Scanner sc = new Scanner(System.in);

		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 ans = new StringBuilder(n * l);

		for (int i = 0; i < n; i++) {
			ans.append(s[i]);
		}

		System.out.println(ans);

		sc.close();
	}

} import java.util.Arrays; Arrays java.util import java.util.Scanner; Scanner 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();

		String[] s = new String[n];
		for (int i = 0; i < n; i++) {
			s[i] = sc.next();
		}

		Arrays.sort(s);

		StringBuilder ans = new StringBuilder(n * l);

		for (int i = 0; i < n; i++) {
			ans.append(s[i]);
		}

		System.out.println(ans);

		sc.close();
	}

} Main public static void main(String[] args) {
		Scanner sc = new Scanner(System.in);

		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 ans = new StringBuilder(n * l);

		for (int i = 0; i < n; i++) {
			ans.append(s[i]);
		}

		System.out.println(ans);

		sc.close();
	} main {
		Scanner sc = new Scanner(System.in);

		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 ans = new StringBuilder(n * l);

		for (int i = 0; i < n; i++) {
			ans.append(s[i]);
		}

		System.out.println(ans);

		sc.close();
	} Scanner sc = new Scanner(System.in); sc new Scanner(System.in) int n = sc.nextInt(); n sc.nextInt() sc.nextInt sc int l = sc.nextInt(); l sc.nextInt() sc.nextInt sc String[] s = new String[n]; s new String[n] n for (int i = 0; i < n; i++) {
			s[i] = sc.next();
		} int i = 0; int i = 0; i 0 i < n i n i++ i++ i {
			s[i] = sc.next();
		} {
			s[i] = sc.next();
		} s[i] = sc.next() s[i] s i sc.next() sc.next sc Arrays.sort(s) Arrays.sort Arrays s StringBuilder ans = new StringBuilder(n * l); ans new StringBuilder(n * l) for (int i = 0; i < n; i++) {
			ans.append(s[i]);
		} int i = 0; int i = 0; i 0 i < n i n i++ i++ i {
			ans.append(s[i]);
		} {
			ans.append(s[i]);
		} ans.append(s[i]) ans.append ans s[i] s i System.out.println(ans) System.out.println System.out System System.out ans sc.close() sc.close sc String[] args args public class Main {

	public static void main(String[] args) {
		Scanner sc = new Scanner(System.in);

		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 ans = new StringBuilder(n * l);

		for (int i = 0; i < n; i++) {
			ans.append(s[i]);
		}

		System.out.println(ans);

		sc.close();
	}

} public class Main {

	public static void main(String[] args) {
		Scanner sc = new Scanner(System.in);

		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 ans = new StringBuilder(n * l);

		for (int i = 0; i < n; i++) {
			ans.append(s[i]);
		}

		System.out.println(ans);

		sc.close();
	}

} Main
import java.util.Arrays;
import java.util.Scanner;

public class Main {

	public static void main(String[] args) {
		Scanner sc = new Scanner(System.in);

		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 ans = new StringBuilder(n * l);

		for (int i = 0; i < n; i++) {
			ans.append(s[i]);
		}

		System.out.println(ans);

		sc.close();
	}

}
