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);
    for(int i=0;i<N;i++){
      System.out.print(S[i]);
    }
      System.out.println();
  }
} 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);
    for(int i=0;i<N;i++){
      System.out.print(S[i]);
    }
      System.out.println();
  }
} 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]);
    }
      System.out.println();
  } 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]);
    }
      System.out.println();
  } 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 System.out.println() System.out.println System.out System System.out 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]);
    }
      System.out.println();
  }
} 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]);
    }
      System.out.println();
  }
} 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);
    for(int i=0;i<N;i++){
      System.out.print(S[i]);
    }
      System.out.println();
  }
}