import java.util.*;
import java.io.BufferedReader; 
import java.io.IOException; 
import java.io.InputStreamReader; 
import java.util.StringTokenizer;

public class Main {
	static class IO 
    { 
        BufferedReader br; 
        StringTokenizer st; 
  
        public IO() 
        { 
            br = new BufferedReader(new
                     InputStreamReader(System.in)); 
        } 
  
        String next() 
        { 
            while (st == null || !st.hasMoreElements()) 
            { 
                try
                { 
                    st = new StringTokenizer(br.readLine()); 
                } 
                catch (IOException  e) 
                { 
                    e.printStackTrace(); 
                } 
            } 
            return st.nextToken(); 
        } 
  
        int nextInt() 
        { 
            return Integer.parseInt(next()); 
        } 
  
        long nextLong() 
        { 
            return Long.parseLong(next()); 
        } 
  
        double nextDouble() 
        { 
            return Double.parseDouble(next()); 
        } 
  
        String nextLine() 
        { 
            String str = ""; 
            try
            { 
                str = br.readLine(); 
            } 
            catch (IOException e) 
            { 
                e.printStackTrace(); 
            } 
            return str; 
        } 
    } 
    public static void main(String[] args){
        IO fs = new IO();
        int n = fs.nextInt(),l = fs.nextInt();
        String [] a = new String[n];
        for(int i = 0; i<n; i++){
            a[i] = fs.nextLine();
        }
        Arrays.sort(a);
        String ans = "";
        for(int i = 0; i<n; i++)ans += a[i];
        System.out.println(ans);
    }
} import java.util.*; util.* java import java.io.BufferedReader; BufferedReader java.io import java.io.IOException; IOException java.io import java.io.InputStreamReader; InputStreamReader java.io import java.util.StringTokenizer; StringTokenizer java.util public class Main {
	static class IO 
    { 
        BufferedReader br; 
        StringTokenizer st; 
  
        public IO() 
        { 
            br = new BufferedReader(new
                     InputStreamReader(System.in)); 
        } 
  
        String next() 
        { 
            while (st == null || !st.hasMoreElements()) 
            { 
                try
                { 
                    st = new StringTokenizer(br.readLine()); 
                } 
                catch (IOException  e) 
                { 
                    e.printStackTrace(); 
                } 
            } 
            return st.nextToken(); 
        } 
  
        int nextInt() 
        { 
            return Integer.parseInt(next()); 
        } 
  
        long nextLong() 
        { 
            return Long.parseLong(next()); 
        } 
  
        double nextDouble() 
        { 
            return Double.parseDouble(next()); 
        } 
  
        String nextLine() 
        { 
            String str = ""; 
            try
            { 
                str = br.readLine(); 
            } 
            catch (IOException e) 
            { 
                e.printStackTrace(); 
            } 
            return str; 
        } 
    } 
    public static void main(String[] args){
        IO fs = new IO();
        int n = fs.nextInt(),l = fs.nextInt();
        String [] a = new String[n];
        for(int i = 0; i<n; i++){
            a[i] = fs.nextLine();
        }
        Arrays.sort(a);
        String ans = "";
        for(int i = 0; i<n; i++)ans += a[i];
        System.out.println(ans);
    }
} Main public static void main(String[] args){
        IO fs = new IO();
        int n = fs.nextInt(),l = fs.nextInt();
        String [] a = new String[n];
        for(int i = 0; i<n; i++){
            a[i] = fs.nextLine();
        }
        Arrays.sort(a);
        String ans = "";
        for(int i = 0; i<n; i++)ans += a[i];
        System.out.println(ans);
    } main {
        IO fs = new IO();
        int n = fs.nextInt(),l = fs.nextInt();
        String [] a = new String[n];
        for(int i = 0; i<n; i++){
            a[i] = fs.nextLine();
        }
        Arrays.sort(a);
        String ans = "";
        for(int i = 0; i<n; i++)ans += a[i];
        System.out.println(ans);
    } IO fs = new IO(); fs new IO() int n = fs.nextInt() n fs.nextInt() fs.nextInt fs l = fs.nextInt(); l fs.nextInt() fs.nextInt fs String [] a = new String[n]; a new String[n] n for(int i = 0; i<n; i++){
            a[i] = fs.nextLine();
        } int i = 0; int i = 0; i 0 i<n i n i++ i++ i {
            a[i] = fs.nextLine();
        } {
            a[i] = fs.nextLine();
        } a[i] = fs.nextLine() a[i] a i fs.nextLine() fs.nextLine fs Arrays.sort(a) Arrays.sort Arrays a String ans = ""; ans "" 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] a i System.out.println(ans) System.out.println System.out System System.out ans String[] args args static class IO 
    { 
        BufferedReader br; 
        StringTokenizer st; 
  
        public IO() 
        { 
            br = new BufferedReader(new
                     InputStreamReader(System.in)); 
        } 
  
        String next() 
        { 
            while (st == null || !st.hasMoreElements()) 
            { 
                try
                { 
                    st = new StringTokenizer(br.readLine()); 
                } 
                catch (IOException  e) 
                { 
                    e.printStackTrace(); 
                } 
            } 
            return st.nextToken(); 
        } 
  
        int nextInt() 
        { 
            return Integer.parseInt(next()); 
        } 
  
        long nextLong() 
        { 
            return Long.parseLong(next()); 
        } 
  
        double nextDouble() 
        { 
            return Double.parseDouble(next()); 
        } 
  
        String nextLine() 
        { 
            String str = ""; 
            try
            { 
                str = br.readLine(); 
            } 
            catch (IOException e) 
            { 
                e.printStackTrace(); 
            } 
            return str; 
        } 
    } IO BufferedReader br; br StringTokenizer st; st public IO() 
        { 
            br = new BufferedReader(new
                     InputStreamReader(System.in)); 
        } IO { 
            br = new BufferedReader(new
                     InputStreamReader(System.in)); 
        } br = new BufferedReader(new
                     InputStreamReader(System.in)) br new BufferedReader(new
                     InputStreamReader(System.in)) String next() 
        { 
            while (st == null || !st.hasMoreElements()) 
            { 
                try
                { 
                    st = new StringTokenizer(br.readLine()); 
                } 
                catch (IOException  e) 
                { 
                    e.printStackTrace(); 
                } 
            } 
            return st.nextToken(); 
        } next { 
            while (st == null || !st.hasMoreElements()) 
            { 
                try
                { 
                    st = new StringTokenizer(br.readLine()); 
                } 
                catch (IOException  e) 
                { 
                    e.printStackTrace(); 
                } 
            } 
            return st.nextToken(); 
        } while (st == null || !st.hasMoreElements()) 
            { 
                try
                { 
                    st = new StringTokenizer(br.readLine()); 
                } 
                catch (IOException  e) 
                { 
                    e.printStackTrace(); 
                } 
            } st == null || !st.hasMoreElements() st == null st null !st.hasMoreElements() st.hasMoreElements() st.hasMoreElements st { 
                try
                { 
                    st = new StringTokenizer(br.readLine()); 
                } 
                catch (IOException  e) 
                { 
                    e.printStackTrace(); 
                } 
            } try
                { 
                    st = new StringTokenizer(br.readLine()); 
                } 
                catch (IOException  e) 
                { 
                    e.printStackTrace(); 
                } catch (IOException  e) 
                { 
                    e.printStackTrace(); 
                } IOException  e { 
                    e.printStackTrace(); 
                } e.printStackTrace() e.printStackTrace e { 
                    st = new StringTokenizer(br.readLine()); 
                } st = new StringTokenizer(br.readLine()) st new StringTokenizer(br.readLine()) return st.nextToken(); st.nextToken() st.nextToken st int nextInt() 
        { 
            return Integer.parseInt(next()); 
        } nextInt { 
            return Integer.parseInt(next()); 
        } return Integer.parseInt(next()); Integer.parseInt(next()) Integer.parseInt Integer next() next long nextLong() 
        { 
            return Long.parseLong(next()); 
        } nextLong { 
            return Long.parseLong(next()); 
        } return Long.parseLong(next()); Long.parseLong(next()) Long.parseLong Long next() next double nextDouble() 
        { 
            return Double.parseDouble(next()); 
        } nextDouble { 
            return Double.parseDouble(next()); 
        } return Double.parseDouble(next()); Double.parseDouble(next()) Double.parseDouble Double next() next String nextLine() 
        { 
            String str = ""; 
            try
            { 
                str = br.readLine(); 
            } 
            catch (IOException e) 
            { 
                e.printStackTrace(); 
            } 
            return str; 
        } nextLine { 
            String str = ""; 
            try
            { 
                str = br.readLine(); 
            } 
            catch (IOException e) 
            { 
                e.printStackTrace(); 
            } 
            return str; 
        } String str = ""; str "" try
            { 
                str = br.readLine(); 
            } 
            catch (IOException e) 
            { 
                e.printStackTrace(); 
            } catch (IOException e) 
            { 
                e.printStackTrace(); 
            } IOException e { 
                e.printStackTrace(); 
            } e.printStackTrace() e.printStackTrace e { 
                str = br.readLine(); 
            } str = br.readLine() str br.readLine() br.readLine br return str; str public class Main {
	static class IO 
    { 
        BufferedReader br; 
        StringTokenizer st; 
  
        public IO() 
        { 
            br = new BufferedReader(new
                     InputStreamReader(System.in)); 
        } 
  
        String next() 
        { 
            while (st == null || !st.hasMoreElements()) 
            { 
                try
                { 
                    st = new StringTokenizer(br.readLine()); 
                } 
                catch (IOException  e) 
                { 
                    e.printStackTrace(); 
                } 
            } 
            return st.nextToken(); 
        } 
  
        int nextInt() 
        { 
            return Integer.parseInt(next()); 
        } 
  
        long nextLong() 
        { 
            return Long.parseLong(next()); 
        } 
  
        double nextDouble() 
        { 
            return Double.parseDouble(next()); 
        } 
  
        String nextLine() 
        { 
            String str = ""; 
            try
            { 
                str = br.readLine(); 
            } 
            catch (IOException e) 
            { 
                e.printStackTrace(); 
            } 
            return str; 
        } 
    } 
    public static void main(String[] args){
        IO fs = new IO();
        int n = fs.nextInt(),l = fs.nextInt();
        String [] a = new String[n];
        for(int i = 0; i<n; i++){
            a[i] = fs.nextLine();
        }
        Arrays.sort(a);
        String ans = "";
        for(int i = 0; i<n; i++)ans += a[i];
        System.out.println(ans);
    }
} public class Main {
	static class IO 
    { 
        BufferedReader br; 
        StringTokenizer st; 
  
        public IO() 
        { 
            br = new BufferedReader(new
                     InputStreamReader(System.in)); 
        } 
  
        String next() 
        { 
            while (st == null || !st.hasMoreElements()) 
            { 
                try
                { 
                    st = new StringTokenizer(br.readLine()); 
                } 
                catch (IOException  e) 
                { 
                    e.printStackTrace(); 
                } 
            } 
            return st.nextToken(); 
        } 
  
        int nextInt() 
        { 
            return Integer.parseInt(next()); 
        } 
  
        long nextLong() 
        { 
            return Long.parseLong(next()); 
        } 
  
        double nextDouble() 
        { 
            return Double.parseDouble(next()); 
        } 
  
        String nextLine() 
        { 
            String str = ""; 
            try
            { 
                str = br.readLine(); 
            } 
            catch (IOException e) 
            { 
                e.printStackTrace(); 
            } 
            return str; 
        } 
    } 
    public static void main(String[] args){
        IO fs = new IO();
        int n = fs.nextInt(),l = fs.nextInt();
        String [] a = new String[n];
        for(int i = 0; i<n; i++){
            a[i] = fs.nextLine();
        }
        Arrays.sort(a);
        String ans = "";
        for(int i = 0; i<n; i++)ans += a[i];
        System.out.println(ans);
    }
} Main
import java.util.*;
import java.io.BufferedReader; 
import java.io.IOException; 
import java.io.InputStreamReader; 
import java.util.StringTokenizer;

public class Main {
	static class IO 
    { 
        BufferedReader br; 
        StringTokenizer st; 
  
        public IO() 
        { 
            br = new BufferedReader(new
                     InputStreamReader(System.in)); 
        } 
  
        String next() 
        { 
            while (st == null || !st.hasMoreElements()) 
            { 
                try
                { 
                    st = new StringTokenizer(br.readLine()); 
                } 
                catch (IOException  e) 
                { 
                    e.printStackTrace(); 
                } 
            } 
            return st.nextToken(); 
        } 
  
        int nextInt() 
        { 
            return Integer.parseInt(next()); 
        } 
  
        long nextLong() 
        { 
            return Long.parseLong(next()); 
        } 
  
        double nextDouble() 
        { 
            return Double.parseDouble(next()); 
        } 
  
        String nextLine() 
        { 
            String str = ""; 
            try
            { 
                str = br.readLine(); 
            } 
            catch (IOException e) 
            { 
                e.printStackTrace(); 
            } 
            return str; 
        } 
    } 
    public static void main(String[] args){
        IO fs = new IO();
        int n = fs.nextInt(),l = fs.nextInt();
        String [] a = new String[n];
        for(int i = 0; i<n; i++){
            a[i] = fs.nextLine();
        }
        Arrays.sort(a);
        String ans = "";
        for(int i = 0; i<n; i++)ans += a[i];
        System.out.println(ans);
    }
}
