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 (String str : s) System.out.print(str);
        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 (String str : s) System.out.print(str);
        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 (String str : s) System.out.print(str);
        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 (String str : s) System.out.print(str);
        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 (String str : s) System.out.print(str); String str s System.out.print(str); System.out.print(str) System.out.print System.out System System.out str 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 (String str : s) System.out.print(str);
        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 (String str : s) System.out.print(str);
        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 (String str : s) System.out.print(str);
        System.out.println();
		
	}	
}