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);
        StringBuilder sb = new StringBuilder();
        for (int i = 0; i < n; i++) {
            sb.append(s[i]);
        }
        System.out.println(sb.toString());
        sc.close();

    }

} 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);
        StringBuilder sb = new StringBuilder();
        for (int i = 0; i < n; i++) {
            sb.append(s[i]);
        }
        System.out.println(sb.toString());
        sc.close();

    }

} 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);
        StringBuilder sb = new StringBuilder();
        for (int i = 0; i < n; i++) {
            sb.append(s[i]);
        }
        System.out.println(sb.toString());
        sc.close();

    } 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);
        StringBuilder sb = new StringBuilder();
        for (int i = 0; i < n; i++) {
            sb.append(s[i]);
        }
        System.out.println(sb.toString());
        sc.close();

    } 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 StringBuilder sb = new StringBuilder(); sb new StringBuilder() for (int i = 0; i < n; i++) {
            sb.append(s[i]);
        } int i = 0; int i = 0; i 0 i < n i n i++ i++ i {
            sb.append(s[i]);
        } {
            sb.append(s[i]);
        } sb.append(s[i]) sb.append sb s[i] s i System.out.println(sb.toString()) System.out.println System.out System System.out sb.toString() sb.toString sb sc.close() sc.close sc 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);
        StringBuilder sb = new StringBuilder();
        for (int i = 0; i < n; i++) {
            sb.append(s[i]);
        }
        System.out.println(sb.toString());
        sc.close();

    }

} 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);
        StringBuilder sb = new StringBuilder();
        for (int i = 0; i < n; i++) {
            sb.append(s[i]);
        }
        System.out.println(sb.toString());
        sc.close();

    }

} 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);
        StringBuilder sb = new StringBuilder();
        for (int i = 0; i < n; i++) {
            sb.append(s[i]);
        }
        System.out.println(sb.toString());
        sc.close();

    }

}
