import java.io.OutputStream;
import java.io.IOException;
import java.io.InputStream;
import java.io.PrintWriter;
import java.util.Arrays;
import java.util.StringTokenizer;
import java.io.IOException;
import java.io.BufferedReader;
import java.io.InputStreamReader;
import java.io.InputStream;

/**
 * Built using CHelper plug-in
 * Actual solution is at the top
 *
 * @author Twinkle Twinkle Little Star
 */
public class Main {
    public static void main(String[] args) {
        InputStream inputStream = System.in;
        OutputStream outputStream = System.out;
        InputReader in = new InputReader(inputStream);
        PrintWriter out = new PrintWriter(outputStream);
        BIrohaLovesStringsABCEdition solver = new BIrohaLovesStringsABCEdition();
        solver.solve(1, in, out);
        out.close();
    }

    static class BIrohaLovesStringsABCEdition {
        public void solve(int testNumber, InputReader in, PrintWriter out) {
            int row = in.nextInt();
            int col = in.nextInt();
            String array[] = new String[row];
            for (int i = 0; i < row; i++) {
                array[i] = in.next();
            }
            Arrays.sort(array);
            String temp = "";
            for (int i = 0; i < row; i++) {
                temp += array[i];
            }
            out.print(temp);
        }

    }

    static class InputReader {
        public BufferedReader reader;
        public StringTokenizer tokenizer;

        public InputReader(InputStream stream) {
            reader = new BufferedReader(new InputStreamReader(stream), 32768);
            tokenizer = null;
        }

        public String next() {
            while (tokenizer == null || !tokenizer.hasMoreTokens()) {
                try {
                    tokenizer = new StringTokenizer(reader.readLine());
                } catch (IOException e) {
                    throw new RuntimeException(e);
                }
            }
            return tokenizer.nextToken();
        }

        public int nextInt() {
            return Integer.parseInt(next());
        }

    }
}
 import java.io.OutputStream; OutputStream java.io import java.io.IOException; IOException java.io import java.io.InputStream; InputStream java.io import java.io.PrintWriter; PrintWriter java.io import java.util.Arrays; Arrays java.util import java.util.StringTokenizer; StringTokenizer java.util import java.io.IOException; IOException java.io import java.io.BufferedReader; BufferedReader java.io import java.io.InputStreamReader; InputStreamReader java.io import java.io.InputStream; InputStream java.io public class Main {
    public static void main(String[] args) {
        InputStream inputStream = System.in;
        OutputStream outputStream = System.out;
        InputReader in = new InputReader(inputStream);
        PrintWriter out = new PrintWriter(outputStream);
        BIrohaLovesStringsABCEdition solver = new BIrohaLovesStringsABCEdition();
        solver.solve(1, in, out);
        out.close();
    }

    static class BIrohaLovesStringsABCEdition {
        public void solve(int testNumber, InputReader in, PrintWriter out) {
            int row = in.nextInt();
            int col = in.nextInt();
            String array[] = new String[row];
            for (int i = 0; i < row; i++) {
                array[i] = in.next();
            }
            Arrays.sort(array);
            String temp = "";
            for (int i = 0; i < row; i++) {
                temp += array[i];
            }
            out.print(temp);
        }

    }

    static class InputReader {
        public BufferedReader reader;
        public StringTokenizer tokenizer;

        public InputReader(InputStream stream) {
            reader = new BufferedReader(new InputStreamReader(stream), 32768);
            tokenizer = null;
        }

        public String next() {
            while (tokenizer == null || !tokenizer.hasMoreTokens()) {
                try {
                    tokenizer = new StringTokenizer(reader.readLine());
                } catch (IOException e) {
                    throw new RuntimeException(e);
                }
            }
            return tokenizer.nextToken();
        }

        public int nextInt() {
            return Integer.parseInt(next());
        }

    }
} Main public static void main(String[] args) {
        InputStream inputStream = System.in;
        OutputStream outputStream = System.out;
        InputReader in = new InputReader(inputStream);
        PrintWriter out = new PrintWriter(outputStream);
        BIrohaLovesStringsABCEdition solver = new BIrohaLovesStringsABCEdition();
        solver.solve(1, in, out);
        out.close();
    } main {
        InputStream inputStream = System.in;
        OutputStream outputStream = System.out;
        InputReader in = new InputReader(inputStream);
        PrintWriter out = new PrintWriter(outputStream);
        BIrohaLovesStringsABCEdition solver = new BIrohaLovesStringsABCEdition();
        solver.solve(1, in, out);
        out.close();
    } InputStream inputStream = System.in; inputStream System.in System System.in OutputStream outputStream = System.out; outputStream System.out System System.out InputReader in = new InputReader(inputStream); in new InputReader(inputStream) PrintWriter out = new PrintWriter(outputStream); out new PrintWriter(outputStream) BIrohaLovesStringsABCEdition solver = new BIrohaLovesStringsABCEdition(); solver new BIrohaLovesStringsABCEdition() solver.solve(1, in, out) solver.solve solver 1 in out out.close() out.close out String[] args args static class BIrohaLovesStringsABCEdition {
        public void solve(int testNumber, InputReader in, PrintWriter out) {
            int row = in.nextInt();
            int col = in.nextInt();
            String array[] = new String[row];
            for (int i = 0; i < row; i++) {
                array[i] = in.next();
            }
            Arrays.sort(array);
            String temp = "";
            for (int i = 0; i < row; i++) {
                temp += array[i];
            }
            out.print(temp);
        }

    } BIrohaLovesStringsABCEdition public void solve(int testNumber, InputReader in, PrintWriter out) {
            int row = in.nextInt();
            int col = in.nextInt();
            String array[] = new String[row];
            for (int i = 0; i < row; i++) {
                array[i] = in.next();
            }
            Arrays.sort(array);
            String temp = "";
            for (int i = 0; i < row; i++) {
                temp += array[i];
            }
            out.print(temp);
        } solve {
            int row = in.nextInt();
            int col = in.nextInt();
            String array[] = new String[row];
            for (int i = 0; i < row; i++) {
                array[i] = in.next();
            }
            Arrays.sort(array);
            String temp = "";
            for (int i = 0; i < row; i++) {
                temp += array[i];
            }
            out.print(temp);
        } int row = in.nextInt(); row in.nextInt() in.nextInt in int col = in.nextInt(); col in.nextInt() in.nextInt in String array[] = new String[row]; array new String[row] row for (int i = 0; i < row; i++) {
                array[i] = in.next();
            } int i = 0; int i = 0; i 0 i < row i row i++ i++ i {
                array[i] = in.next();
            } {
                array[i] = in.next();
            } array[i] = in.next() array[i] array i in.next() in.next in Arrays.sort(array) Arrays.sort Arrays array String temp = ""; temp "" for (int i = 0; i < row; i++) {
                temp += array[i];
            } int i = 0; int i = 0; i 0 i < row i row i++ i++ i {
                temp += array[i];
            } {
                temp += array[i];
            } temp += array[i] temp array[i] array i out.print(temp) out.print out temp int testNumber testNumber InputReader in in PrintWriter out out static class InputReader {
        public BufferedReader reader;
        public StringTokenizer tokenizer;

        public InputReader(InputStream stream) {
            reader = new BufferedReader(new InputStreamReader(stream), 32768);
            tokenizer = null;
        }

        public String next() {
            while (tokenizer == null || !tokenizer.hasMoreTokens()) {
                try {
                    tokenizer = new StringTokenizer(reader.readLine());
                } catch (IOException e) {
                    throw new RuntimeException(e);
                }
            }
            return tokenizer.nextToken();
        }

        public int nextInt() {
            return Integer.parseInt(next());
        }

    } InputReader public BufferedReader reader; reader public StringTokenizer tokenizer; tokenizer public InputReader(InputStream stream) {
            reader = new BufferedReader(new InputStreamReader(stream), 32768);
            tokenizer = null;
        } InputReader {
            reader = new BufferedReader(new InputStreamReader(stream), 32768);
            tokenizer = null;
        } reader = new BufferedReader(new InputStreamReader(stream), 32768) reader new BufferedReader(new InputStreamReader(stream), 32768) tokenizer = null tokenizer null InputStream stream stream public String next() {
            while (tokenizer == null || !tokenizer.hasMoreTokens()) {
                try {
                    tokenizer = new StringTokenizer(reader.readLine());
                } catch (IOException e) {
                    throw new RuntimeException(e);
                }
            }
            return tokenizer.nextToken();
        } next {
            while (tokenizer == null || !tokenizer.hasMoreTokens()) {
                try {
                    tokenizer = new StringTokenizer(reader.readLine());
                } catch (IOException e) {
                    throw new RuntimeException(e);
                }
            }
            return tokenizer.nextToken();
        } while (tokenizer == null || !tokenizer.hasMoreTokens()) {
                try {
                    tokenizer = new StringTokenizer(reader.readLine());
                } catch (IOException e) {
                    throw new RuntimeException(e);
                }
            } tokenizer == null || !tokenizer.hasMoreTokens() tokenizer == null tokenizer null !tokenizer.hasMoreTokens() tokenizer.hasMoreTokens() tokenizer.hasMoreTokens tokenizer {
                try {
                    tokenizer = new StringTokenizer(reader.readLine());
                } catch (IOException e) {
                    throw new RuntimeException(e);
                }
            } try {
                    tokenizer = new StringTokenizer(reader.readLine());
                } catch (IOException e) {
                    throw new RuntimeException(e);
                } catch (IOException e) {
                    throw new RuntimeException(e);
                } IOException e {
                    throw new RuntimeException(e);
                } throw new RuntimeException(e); new RuntimeException(e) {
                    tokenizer = new StringTokenizer(reader.readLine());
                } tokenizer = new StringTokenizer(reader.readLine()) tokenizer new StringTokenizer(reader.readLine()) return tokenizer.nextToken(); tokenizer.nextToken() tokenizer.nextToken tokenizer public int nextInt() {
            return Integer.parseInt(next());
        } nextInt {
            return Integer.parseInt(next());
        } return Integer.parseInt(next()); Integer.parseInt(next()) Integer.parseInt Integer next() next public class Main {
    public static void main(String[] args) {
        InputStream inputStream = System.in;
        OutputStream outputStream = System.out;
        InputReader in = new InputReader(inputStream);
        PrintWriter out = new PrintWriter(outputStream);
        BIrohaLovesStringsABCEdition solver = new BIrohaLovesStringsABCEdition();
        solver.solve(1, in, out);
        out.close();
    }

    static class BIrohaLovesStringsABCEdition {
        public void solve(int testNumber, InputReader in, PrintWriter out) {
            int row = in.nextInt();
            int col = in.nextInt();
            String array[] = new String[row];
            for (int i = 0; i < row; i++) {
                array[i] = in.next();
            }
            Arrays.sort(array);
            String temp = "";
            for (int i = 0; i < row; i++) {
                temp += array[i];
            }
            out.print(temp);
        }

    }

    static class InputReader {
        public BufferedReader reader;
        public StringTokenizer tokenizer;

        public InputReader(InputStream stream) {
            reader = new BufferedReader(new InputStreamReader(stream), 32768);
            tokenizer = null;
        }

        public String next() {
            while (tokenizer == null || !tokenizer.hasMoreTokens()) {
                try {
                    tokenizer = new StringTokenizer(reader.readLine());
                } catch (IOException e) {
                    throw new RuntimeException(e);
                }
            }
            return tokenizer.nextToken();
        }

        public int nextInt() {
            return Integer.parseInt(next());
        }

    }
} public class Main {
    public static void main(String[] args) {
        InputStream inputStream = System.in;
        OutputStream outputStream = System.out;
        InputReader in = new InputReader(inputStream);
        PrintWriter out = new PrintWriter(outputStream);
        BIrohaLovesStringsABCEdition solver = new BIrohaLovesStringsABCEdition();
        solver.solve(1, in, out);
        out.close();
    }

    static class BIrohaLovesStringsABCEdition {
        public void solve(int testNumber, InputReader in, PrintWriter out) {
            int row = in.nextInt();
            int col = in.nextInt();
            String array[] = new String[row];
            for (int i = 0; i < row; i++) {
                array[i] = in.next();
            }
            Arrays.sort(array);
            String temp = "";
            for (int i = 0; i < row; i++) {
                temp += array[i];
            }
            out.print(temp);
        }

    }

    static class InputReader {
        public BufferedReader reader;
        public StringTokenizer tokenizer;

        public InputReader(InputStream stream) {
            reader = new BufferedReader(new InputStreamReader(stream), 32768);
            tokenizer = null;
        }

        public String next() {
            while (tokenizer == null || !tokenizer.hasMoreTokens()) {
                try {
                    tokenizer = new StringTokenizer(reader.readLine());
                } catch (IOException e) {
                    throw new RuntimeException(e);
                }
            }
            return tokenizer.nextToken();
        }

        public int nextInt() {
            return Integer.parseInt(next());
        }

    }
} Main
import java.io.OutputStream;
import java.io.IOException;
import java.io.InputStream;
import java.io.PrintWriter;
import java.util.Arrays;
import java.util.StringTokenizer;
import java.io.IOException;
import java.io.BufferedReader;
import java.io.InputStreamReader;
import java.io.InputStream;

/**
 * Built using CHelper plug-in
 * Actual solution is at the top
 *
 * @author Twinkle Twinkle Little Star
 */
public class Main {
    public static void main(String[] args) {
        InputStream inputStream = System.in;
        OutputStream outputStream = System.out;
        InputReader in = new InputReader(inputStream);
        PrintWriter out = new PrintWriter(outputStream);
        BIrohaLovesStringsABCEdition solver = new BIrohaLovesStringsABCEdition();
        solver.solve(1, in, out);
        out.close();
    }

    static class BIrohaLovesStringsABCEdition {
        public void solve(int testNumber, InputReader in, PrintWriter out) {
            int row = in.nextInt();
            int col = in.nextInt();
            String array[] = new String[row];
            for (int i = 0; i < row; i++) {
                array[i] = in.next();
            }
            Arrays.sort(array);
            String temp = "";
            for (int i = 0; i < row; i++) {
                temp += array[i];
            }
            out.print(temp);
        }

    }

    static class InputReader {
        public BufferedReader reader;
        public StringTokenizer tokenizer;

        public InputReader(InputStream stream) {
            reader = new BufferedReader(new InputStreamReader(stream), 32768);
            tokenizer = null;
        }

        public String next() {
            while (tokenizer == null || !tokenizer.hasMoreTokens()) {
                try {
                    tokenizer = new StringTokenizer(reader.readLine());
                } catch (IOException e) {
                    throw new RuntimeException(e);
                }
            }
            return tokenizer.nextToken();
        }

        public int nextInt() {
            return Integer.parseInt(next());
        }

    }
}

