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();
		sc.nextLine();
		String[] s = new String[N];
		for(int i = 0; i < N; i++) {
			s[i] = sc.nextLine();
		}
		sc.close();
		Arrays.sort(s);
		for(int i = 0; i < N; i++) {
			System.out.print(s[i]);
		}
		System.out.println("");
	}
} 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();
		sc.nextLine();
		String[] s = new String[N];
		for(int i = 0; i < N; i++) {
			s[i] = sc.nextLine();
		}
		sc.close();
		Arrays.sort(s);
		for(int i = 0; i < N; i++) {
			System.out.print(s[i]);
		}
		System.out.println("");
	}
} Main public static void main(String[] args)  {
		Scanner sc = new Scanner(System.in);
		int N = sc.nextInt();
		int L = sc.nextInt();
		sc.nextLine();
		String[] s = new String[N];
		for(int i = 0; i < N; i++) {
			s[i] = sc.nextLine();
		}
		sc.close();
		Arrays.sort(s);
		for(int i = 0; i < N; i++) {
			System.out.print(s[i]);
		}
		System.out.println("");
	} main {
		Scanner sc = new Scanner(System.in);
		int N = sc.nextInt();
		int L = sc.nextInt();
		sc.nextLine();
		String[] s = new String[N];
		for(int i = 0; i < N; i++) {
			s[i] = sc.nextLine();
		}
		sc.close();
		Arrays.sort(s);
		for(int i = 0; i < N; i++) {
			System.out.print(s[i]);
		}
		System.out.println("");
	} 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 sc.nextLine() sc.nextLine sc String[] s = new String[N]; s new String[N] N for(int i = 0; i < N; i++) {
			s[i] = sc.nextLine();
		} int i = 0; int i = 0; i 0 i < N i N i++ i++ i {
			s[i] = sc.nextLine();
		} {
			s[i] = sc.nextLine();
		} s[i] = sc.nextLine() s[i] s i sc.nextLine() sc.nextLine sc sc.close() sc.close sc Arrays.sort(s) Arrays.sort Arrays s for(int i = 0; i < N; i++) {
			System.out.print(s[i]);
		} int i = 0; int i = 0; i 0 i < N i N i++ i++ i {
			System.out.print(s[i]);
		} {
			System.out.print(s[i]);
		} System.out.print(s[i]) System.out.print System.out System System.out s[i] s i System.out.println("") System.out.println System.out System System.out "" 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();
		sc.nextLine();
		String[] s = new String[N];
		for(int i = 0; i < N; i++) {
			s[i] = sc.nextLine();
		}
		sc.close();
		Arrays.sort(s);
		for(int i = 0; i < N; i++) {
			System.out.print(s[i]);
		}
		System.out.println("");
	}
} public class Main {
	public static void main(String[] args)  {
		Scanner sc = new Scanner(System.in);
		int N = sc.nextInt();
		int L = sc.nextInt();
		sc.nextLine();
		String[] s = new String[N];
		for(int i = 0; i < N; i++) {
			s[i] = sc.nextLine();
		}
		sc.close();
		Arrays.sort(s);
		for(int i = 0; i < N; i++) {
			System.out.print(s[i]);
		}
		System.out.println("");
	}
} 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();
		sc.nextLine();
		String[] s = new String[N];
		for(int i = 0; i < N; i++) {
			s[i] = sc.nextLine();
		}
		sc.close();
		Arrays.sort(s);
		for(int i = 0; i < N; i++) {
			System.out.print(s[i]);
		}
		System.out.println("");
	}
}