import java.util.Scanner;
import java.util.Arrays;
import java.util.Collections;
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(int i = 0;i < N;i++){
      System.out.print(s[i]);
    }
  }
} import java.util.Scanner; Scanner java.util import java.util.Arrays; Arrays java.util import java.util.Collections; Collections 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(int i = 0;i < N;i++){
      System.out.print(s[i]);
    }
  }
} 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(int i = 0;i < N;i++){
      System.out.print(s[i]);
    }
  } 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(int i = 0;i < N;i++){
      System.out.print(s[i]);
    }
  } 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(int i = 0;i < N;i++){
      System.out.print(s[i]);
    } int i = 0; int i = 0; i 0 i < N i N i++ i++ i {
      System.out.print(s[i]);
    } {
      System.out.print(s[i]);
    } System.out.print(s[i]) System.out.print System.out System System.out s[i] s i 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(int i = 0;i < N;i++){
      System.out.print(s[i]);
    }
  }
} 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(int i = 0;i < N;i++){
      System.out.print(s[i]);
    }
  }
} Main
import java.util.Scanner;
import java.util.Arrays;
import java.util.Collections;
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(int i = 0;i < N;i++){
      System.out.print(s[i]);
    }
  }
}
