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 A[] =new String[N];
		for(int i=0;i<N;i++){
            A[i]=sc.next();
            
        }
		String ans="";
		
		Arrays.sort(A);
		
		for(int i=0;i<N;i++){
            ans+=A[i];
            
        }
        
        
        System.out.println(ans);
		
		
		
	}
} 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 A[] =new String[N];
		for(int i=0;i<N;i++){
            A[i]=sc.next();
            
        }
		String ans="";
		
		Arrays.sort(A);
		
		for(int i=0;i<N;i++){
            ans+=A[i];
            
        }
        
        
        System.out.println(ans);
		
		
		
	}
} Main public static void main(String[] args){
		Scanner sc = new Scanner(System.in);
		int N = sc.nextInt();
		int L = sc.nextInt();
		
		String A[] =new String[N];
		for(int i=0;i<N;i++){
            A[i]=sc.next();
            
        }
		String ans="";
		
		Arrays.sort(A);
		
		for(int i=0;i<N;i++){
            ans+=A[i];
            
        }
        
        
        System.out.println(ans);
		
		
		
	} main {
		Scanner sc = new Scanner(System.in);
		int N = sc.nextInt();
		int L = sc.nextInt();
		
		String A[] =new String[N];
		for(int i=0;i<N;i++){
            A[i]=sc.next();
            
        }
		String ans="";
		
		Arrays.sort(A);
		
		for(int i=0;i<N;i++){
            ans+=A[i];
            
        }
        
        
        System.out.println(ans);
		
		
		
	} 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 A[] =new String[N]; A new String[N] N for(int i=0;i<N;i++){
            A[i]=sc.next();
            
        } int i=0; int i=0; i 0 i<N i N i++ i++ i {
            A[i]=sc.next();
            
        } {
            A[i]=sc.next();
            
        } A[i]=sc.next() A[i] A i sc.next() sc.next sc String ans=""; ans "" Arrays.sort(A) Arrays.sort Arrays A for(int i=0;i<N;i++){
            ans+=A[i];
            
        } int i=0; int i=0; i 0 i<N i N i++ i++ i {
            ans+=A[i];
            
        } {
            ans+=A[i];
            
        } ans+=A[i] ans A[i] A i System.out.println(ans) System.out.println System.out System System.out ans 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 A[] =new String[N];
		for(int i=0;i<N;i++){
            A[i]=sc.next();
            
        }
		String ans="";
		
		Arrays.sort(A);
		
		for(int i=0;i<N;i++){
            ans+=A[i];
            
        }
        
        
        System.out.println(ans);
		
		
		
	}
} public class Main {
	public static void main(String[] args){
		Scanner sc = new Scanner(System.in);
		int N = sc.nextInt();
		int L = sc.nextInt();
		
		String A[] =new String[N];
		for(int i=0;i<N;i++){
            A[i]=sc.next();
            
        }
		String ans="";
		
		Arrays.sort(A);
		
		for(int i=0;i<N;i++){
            ans+=A[i];
            
        }
        
        
        System.out.println(ans);
		
		
		
	}
} 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 A[] =new String[N];
		for(int i=0;i<N;i++){
            A[i]=sc.next();
            
        }
		String ans="";
		
		Arrays.sort(A);
		
		for(int i=0;i<N;i++){
            ans+=A[i];
            
        }
        
        
        System.out.println(ans);
		
		
		
	}
}
