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);
    String m="";
    for(int i=0;i<n;i++) m+=s[i];
    System.out.println(m);
  }
} 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);
    String m="";
    for(int i=0;i<n;i++) m+=s[i];
    System.out.println(m);
  }
} 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);
    String m="";
    for(int i=0;i<n;i++) m+=s[i];
    System.out.println(m);
  } 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);
    String m="";
    for(int i=0;i<n;i++) m+=s[i];
    System.out.println(m);
  } 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 String m=""; m "" for(int i=0;i<n;i++) m+=s[i]; int i=0; int i=0; i 0 i<n i n i++ i++ i m+=s[i]; m+=s[i] m s[i] s i System.out.println(m) System.out.println System.out System System.out m 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);
    String m="";
    for(int i=0;i<n;i++) m+=s[i];
    System.out.println(m);
  }
} 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);
    String m="";
    for(int i=0;i<n;i++) m+=s[i];
    System.out.println(m);
  }
} 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);
    String m="";
    for(int i=0;i<n;i++) m+=s[i];
    System.out.println(m);
  }
}