import 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);
		for(int i = 0; i < N; i ++){
			System.out.print(S[i]);
		}


	}
} import java.util.*; util.* java 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);
		for(int i = 0; i < N; i ++){
			System.out.print(S[i]);
		}


	}
} 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);
		for(int i = 0; i < N; i ++){
			System.out.print(S[i]);
		}


	} 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);
		for(int i = 0; i < N; i ++){
			System.out.print(S[i]);
		}


	} 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 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 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);
		for(int i = 0; i < N; i ++){
			System.out.print(S[i]);
		}


	}
} 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);
		for(int i = 0; i < N; i ++){
			System.out.print(S[i]);
		}


	}
} Main
import 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);
		for(int i = 0; i < N; i ++){
			System.out.print(S[i]);
		}


	}
}