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

public class Main {

	private static Scanner sc = new Scanner(System.in);

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

	private static Scanner sc = new Scanner(System.in);

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

	private static Scanner sc = new Scanner(System.in);

	public static void main(String[] args) {
		int n = sc.nextInt(), l = sc.nextInt();
		String[] s = new String[n];
		for (int i = 0; i < n; i++) {
			s[i] = sc.next();
		}
		Arrays.sort(s);
		StringBuilder sb = new StringBuilder();
		for (int i = 0; i < n; i++) {
			sb.append(s[i]);
		}
		System.out.println(sb);
	}
} public class Main {

	private static Scanner sc = new Scanner(System.in);

	public static void main(String[] args) {
		int n = sc.nextInt(), l = sc.nextInt();
		String[] s = new String[n];
		for (int i = 0; i < n; i++) {
			s[i] = sc.next();
		}
		Arrays.sort(s);
		StringBuilder sb = new StringBuilder();
		for (int i = 0; i < n; i++) {
			sb.append(s[i]);
		}
		System.out.println(sb);
	}
} Main
import java.util.Arrays;
import java.util.Scanner;

public class Main {

	private static Scanner sc = new Scanner(System.in);

	public static void main(String[] args) {
		int n = sc.nextInt(), l = sc.nextInt();
		String[] s = new String[n];
		for (int i = 0; i < n; i++) {
			s[i] = sc.next();
		}
		Arrays.sort(s);
		StringBuilder sb = new StringBuilder();
		for (int i = 0; i < n; i++) {
			sb.append(s[i]);
		}
		System.out.println(sb);
	}
}
