
import java.util.*;

public class Main {
  public static void main(String[] args) {
    @SuppressWarnings("resource")
    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 (String s : S) {
      sb.append(s);
    }

    System.out.println(sb.toString());
  }
} import java.util.*; util.* java public class Main {
  public static void main(String[] args) {
    @SuppressWarnings("resource")
    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 (String s : S) {
      sb.append(s);
    }

    System.out.println(sb.toString());
  }
} Main public static void main(String[] args) {
    @SuppressWarnings("resource")
    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 (String s : S) {
      sb.append(s);
    }

    System.out.println(sb.toString());
  } main {
    @SuppressWarnings("resource")
    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 (String s : S) {
      sb.append(s);
    }

    System.out.println(sb.toString());
  } @SuppressWarnings("resource")
    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 (String s : S) {
      sb.append(s);
    } String s S {
      sb.append(s);
    } {
      sb.append(s);
    } sb.append(s) sb.append sb s 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) {
    @SuppressWarnings("resource")
    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 (String s : S) {
      sb.append(s);
    }

    System.out.println(sb.toString());
  }
} public class Main {
  public static void main(String[] args) {
    @SuppressWarnings("resource")
    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 (String s : S) {
      sb.append(s);
    }

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

import java.util.*;

public class Main {
  public static void main(String[] args) {
    @SuppressWarnings("resource")
    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 (String s : S) {
      sb.append(s);
    }

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