

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 [] str = new String[N];

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

        Arrays.sort(str);

        StringBuilder sb = new StringBuilder();

        for(int i = 0; i < N; i++) {
            sb.append(str[i]);
        }

        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();
        String [] str = new String[N];

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

        Arrays.sort(str);

        StringBuilder sb = new StringBuilder();

        for(int i = 0; i < N; i++) {
            sb.append(str[i]);
        }

        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();
        String [] str = new String[N];

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

        Arrays.sort(str);

        StringBuilder sb = new StringBuilder();

        for(int i = 0; i < N; i++) {
            sb.append(str[i]);
        }

        System.out.println(sb.toString());
    } main {
        Scanner sc = new Scanner(System.in);
        int N = sc.nextInt();
        int L = sc.nextInt();
        String [] str = new String[N];

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

        Arrays.sort(str);

        StringBuilder sb = new StringBuilder();

        for(int i = 0; i < N; i++) {
            sb.append(str[i]);
        }

        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 String [] str = new String[N]; str new String[N] N for(int i = 0; i < N; i++) {
            str[i] = sc.next();
        } int i = 0; int i = 0; i 0 i < N i N i++ i++ i {
            str[i] = sc.next();
        } {
            str[i] = sc.next();
        } str[i] = sc.next() str[i] str i sc.next() sc.next sc Arrays.sort(str) Arrays.sort Arrays str StringBuilder sb = new StringBuilder(); sb new StringBuilder() for(int i = 0; i < N; i++) {
            sb.append(str[i]);
        } int i = 0; int i = 0; i 0 i < N i N i++ i++ i {
            sb.append(str[i]);
        } {
            sb.append(str[i]);
        } sb.append(str[i]) sb.append sb str[i] str i 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();
        String [] str = new String[N];

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

        Arrays.sort(str);

        StringBuilder sb = new StringBuilder();

        for(int i = 0; i < N; i++) {
            sb.append(str[i]);
        }

        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();
        String [] str = new String[N];

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

        Arrays.sort(str);

        StringBuilder sb = new StringBuilder();

        for(int i = 0; i < N; i++) {
            sb.append(str[i]);
        }

        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();
        String [] str = new String[N];

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

        Arrays.sort(str);

        StringBuilder sb = new StringBuilder();

        for(int i = 0; i < N; i++) {
            sb.append(str[i]);
        }

        System.out.println(sb.toString());
    }
}
