//
import java.math.*;
import java.util.*;
import java.io.*;
  
public class Main { 
  
    static BufferedReader in;
    static PrintWriter out = new PrintWriter(System.out);
    static String file = "../in";
    static int test = 10; // 0 for local testing, 1 for std input
    static int inf = 1_000_000;

    void swap(int[]ary, int i, int j)
    {
        int t = ary[i];
        ary[i] = ary[j];
        ary[j] = t;
    }
    
    String[] split() throws Exception
    {
        return in.readLine().split(" ");
    }

    int readInt() throws Exception
    {
        return Integer.valueOf(in.readLine());
    }

    int[] toIntArray() throws Exception
    {
        String[] sp = split();
        int n = sp.length;
        int[] ary = new int[n];
        for(int i = 0; i < n; i++) ary[i] = Integer.valueOf(sp[i]);
        return ary;
    }

    String reverse(String str)
    {
        return new StringBuilder(str).reverse().toString();
    }

    public static void main(String[] args) throws Exception
    {
        int _k = Integer.valueOf("1");
        if(test > 0) in = new BufferedReader(new InputStreamReader(System.in));
        else in = new BufferedReader(new FileReader(file));
        if(test < 0) {String[] str = in.readLine().split(" ");}
/***********************************************************************/
/***********************************************************************/
/***********************************************************************/
/***********************************************************************/

        new Main().b();
        out.flush();
    }
    
    int mod = 1_000_000_007;

    long pow(long a, int pow)
    {
        long res = 1;
        while(pow > 0)
        {
            if(pow % 2 != 0) {
                res *= a;
                res %= mod;
            }
            a = a * a;
            a %= mod;
            pow /= 2;
        }

        return res;
    }

    long comb(int n, int k)
    {
        if(n < 0 || k < 0 || k > n) return 0;

        return (((f[n] * ff[k]) % mod) * f[n - k]) % mod;
    }

    long[] f = new long[2_000_010], ff = new long[2_000_010];

    void f() throws Exception
    {
        
    }

    void e2(int n, int k)
    {
        for(int i = 0; i <= n; i++)
        {
            int a = i;
            int c = 0;
            while(a > 0)
            {
                if(a % 10 != 0) c++;
                a /= 10;
            }
            if(c == k) System.out.println(i);
        }
    }

    long ehelper(String str, int k)
    {
        int idx = 0;
        while(str.charAt(idx) == '0') idx++;
        str = str.substring(idx);
        char[] chs = str.toCharArray();
        int n = chs.length;
        if(n < k)
        {
            // System.out.println(0);
            return 0;
        }
        long sum = 0;

        if(k == 1)
        {
            sum += chs[0] - '0';
            sum += (n - 1) * 9;
        }
        if(k == 2)
        {
            int val = chs[0] - '0';
            sum += (n - 1) * (n - 2) / 2 * 81;
            for(int i = 1; i < val; i++)
            {
                sum += (n - 1) * 9;
            }
            for(int i = 1; i < n; i++) {
                if(chs[i] != '0')
                {
                    sum += chs[i] - '0';
                    sum += 9 * (n - i - 1);
                    break;
                }
            }
        }
        if(k == 3)
        {
            int val = chs[0] - '0';
            sum += (n - 1) * (n - 2) * (n - 3) / 6 * 729;
            for(int i = 1; i < val; i++)
            {
                sum += (n - 1) * (n - 2) / 2 * 81;
            }
            // System.out.println("..." +sum);
            sum += ehelper(str.substring(1), k - 1);
        }
        
        return sum;
    }


    void e() throws Exception
    {
        String str = in.readLine();
        int k = readInt();
        
        // e2(Integer.valueOf(str), k);
        System.out.println(ehelper(str, k));
    }

    long gcd(long a, long b)
    {
        if(b == 0) return a;
        return gcd(b, a % b);
    }

    void d() throws Exception
    {
        int[] ary = toIntArray();
        int n = ary[0], k = ary[1];
        ary = toIntArray();
        
        Arrays.sort(ary);
        long lo = Long.MIN_VALUE,
                hi = Long.MAX_VALUE;

        while(lo < hi)
        {
            long m = (lo + hi) / 2;
            if(lo < 0) m--;
            
            int c = bs(ary, m);
            // System.out.printf("%d %d %d %d\n", lo, hi, m, c);
            if(c < k) {
                // if(lo < 0 && hi < 0)
                lo = m + 1;
            }
            else {
                if(lo < 0 && hi < 0) hi = m - 1;
                else hi = m;
            }
        }
        out.println(lo);
    }

    int bs(int[] ary, long t)
    {
        int m = ary.length;
        int n = m;
        int count = 0;
        for(int i = 0; i < m - 1; i++)
        {
            // for each row
            // System.out.println(i);
            int lo = i + 1, hi = n - 1;
            long p1 = (long) ary[i];
            while(lo < hi)
            {
                if(p1 > 0)
                {
                    int mid = (lo + hi) / 2;
                    long prod = p1 * ary[mid];
                    if(prod < t) lo = mid + 1;
                    else hi = mid;
                }
                else
                {
                    int mid = (lo + hi + 1) / 2;
                    long prod = p1 * ary[mid];
                    if(prod < t) hi = mid - 1;
                    else lo = mid;
                }

            }
            
            if(p1 == 0)
            {
                if(t > 0) count += n - i - 1;
            }
            else if(p1 < 0) {
                if(p1 * ary[lo] >= t) lo++;
                count += n - lo;
                // System.out.printf("%d %d %d \n", ary[i], lo, count);
            }
            else {  // >= 0
                if(ary[lo] * p1 > t) lo--;
                else if(ary[lo] * p1 < t) lo++;
                count += lo - i;
// System.out.printf("%d %d %d\n", ary[i], i, lo - i);                
            }
        }
        return count;
    }
    
    void c() throws Exception
    {
        int n = readInt();
        TreeMap<String, Integer> map = new TreeMap<>();
        int maxi = 0;

        for(int i = 0; i< n; i++)
        {
            String str = in.readLine();
            map.put(str, map.getOrDefault(str, 0) + 1);
            maxi = Math.max(maxi, map.get(str));
        }

        for(String str : map.keySet())
        {
            if(map.get(str) == maxi)
                out.println(str);
        }
    }

    void b() throws Exception
    {
        int[] ary = toIntArray();
        int n = ary[0], l = ary[1];
        String[] str = new String[n];

        for(int i = 0; i < n; i++)
        {
            str[i] = in.readLine();
        }
        
        Arrays.sort(str);

        StringBuilder sb = new StringBuilder();
        for(String s : str) sb.append(s);
        out.println(sb);
    }

    void a() throws Exception
    {
        int[] ary = toIntArray();
        Arrays.sort(ary);
        if(ary[0] == 5 && ary[1] == 5 && ary[2] == 7)
        {
            out.println("YES");
        }
        else out.println("NO");
    }
}

Measured memory_bytes: 54612