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();
		if(L == 0);
		String[]S = new String[N];
		for(int i = 0; i < N; i++) {
			S[i] = sc.next();
		}
		sc.close();
		Arrays.sort(S);
		StringBuilder sb = new StringBuilder();
		for(String t : S) {
			sb.append(t);
		}
		System.out.println(sb.toString());
	}
} 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();
		if(L == 0);
		String[]S = new String[N];
		for(int i = 0; i < N; i++) {
			S[i] = sc.next();
		}
		sc.close();
		Arrays.sort(S);
		StringBuilder sb = new StringBuilder();
		for(String t : S) {
			sb.append(t);
		}
		System.out.println(sb.toString());
	}
} Main public static void main(String[] args) {
		Scanner sc = new Scanner(System.in);
		int N = sc.nextInt();
		int L = sc.nextInt();
		if(L == 0);
		String[]S = new String[N];
		for(int i = 0; i < N; i++) {
			S[i] = sc.next();
		}
		sc.close();
		Arrays.sort(S);
		StringBuilder sb = new StringBuilder();
		for(String t : S) {
			sb.append(t);
		}
		System.out.println(sb.toString());
	} main {
		Scanner sc = new Scanner(System.in);
		int N = sc.nextInt();
		int L = sc.nextInt();
		if(L == 0);
		String[]S = new String[N];
		for(int i = 0; i < N; i++) {
			S[i] = sc.next();
		}
		sc.close();
		Arrays.sort(S);
		StringBuilder sb = new StringBuilder();
		for(String t : S) {
			sb.append(t);
		}
		System.out.println(sb.toString());
	} 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 if(L == 0); L == 0 L 0 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 sc.close() sc.close sc Arrays.sort(S) Arrays.sort Arrays S StringBuilder sb = new StringBuilder(); sb new StringBuilder() for(String t : S) {
			sb.append(t);
		} String t S {
			sb.append(t);
		} {
			sb.append(t);
		} sb.append(t) sb.append sb t System.out.println(sb.toString()) System.out.println System.out System System.out sb.toString() sb.toString sb 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();
		if(L == 0);
		String[]S = new String[N];
		for(int i = 0; i < N; i++) {
			S[i] = sc.next();
		}
		sc.close();
		Arrays.sort(S);
		StringBuilder sb = new StringBuilder();
		for(String t : S) {
			sb.append(t);
		}
		System.out.println(sb.toString());
	}
} public class Main {
	public static void main(String[] args) {
		Scanner sc = new Scanner(System.in);
		int N = sc.nextInt();
		int L = sc.nextInt();
		if(L == 0);
		String[]S = new String[N];
		for(int i = 0; i < N; i++) {
			S[i] = sc.next();
		}
		sc.close();
		Arrays.sort(S);
		StringBuilder sb = new StringBuilder();
		for(String t : S) {
			sb.append(t);
		}
		System.out.println(sb.toString());
	}
} 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();
		if(L == 0);
		String[]S = new String[N];
		for(int i = 0; i < N; i++) {
			S[i] = sc.next();
		}
		sc.close();
		Arrays.sort(S);
		StringBuilder sb = new StringBuilder();
		for(String t : S) {
			sb.append(t);
		}
		System.out.println(sb.toString());
	}
}
