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