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[] s = new String[n];

        for (int i = 0; i < n; i++) {
            s[i] = sc.next();
        }

        Arrays.sort(s);
        
        for (String s1 : s)
            System.out.print(s1);
    }
} 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[] s = new String[n];

        for (int i = 0; i < n; i++) {
            s[i] = sc.next();
        }

        Arrays.sort(s);
        
        for (String s1 : s)
            System.out.print(s1);
    }
} 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 (String s1 : s)
            System.out.print(s1);
    } 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 (String s1 : s)
            System.out.print(s1);
    } 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 (String s1 : s)
            System.out.print(s1); String s1 s System.out.print(s1); System.out.print(s1) System.out.print System.out System System.out s1 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 (String s1 : s)
            System.out.print(s1);
    }
} 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 (String s1 : s)
            System.out.print(s1);
    }
} 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[] s = new String[n];

        for (int i = 0; i < n; i++) {
            s[i] = sc.next();
        }

        Arrays.sort(s);
        
        for (String s1 : s)
            System.out.print(s1);
    }
}