
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);
    }
} 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);
    }
} 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);
    } 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);
    } 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) System.out.println System.out System System.out 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[] 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);
    }
} 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);
    }
} 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);
    }
}