import java.io.IOException;
import java.io.InputStream;
import java.util.Arrays;
import java.util.NoSuchElementException;


public class Main {
    public static void main(String[] args) {
        FastScanner sc = new FastScanner();
        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();
    }
}


class FastScanner {
    public static String debug = null;

    private final InputStream in = System.in;
    private int ptr = 0;
    private int buflen = 0;
    private byte[] buffer = new byte[1024];
    private boolean eos = false;

    private boolean hasNextByte() {
        if (ptr < buflen) {
            return true;
        } else {
            ptr = 0;
            try {
                if (debug != null) {
                    buflen = debug.length();
                    buffer = debug.getBytes();
                    debug = "";
                    eos = true;
                } else {
                    buflen = in.read(buffer);
                }
            } catch (IOException e) {
                e.printStackTrace();
            }
            if (buflen < 0) {
                eos = true;
                return false;
            } else if (buflen == 0) {
                return false;
            }
        }
        return true;
    }

    private int readByte() {
        if (hasNextByte())
            return buffer[ptr++];
        else
            return -1;
    }

    private static boolean isPrintableChar(int c) {
        return 33 <= c && c <= 126;
    }

    private void skipUnprintable() {
        while (hasNextByte() && !isPrintableChar(buffer[ptr]))
            ptr++;
    }

    public boolean isEOS() {
        return this.eos;
    }

    public boolean hasNext() {
        skipUnprintable();
        return hasNextByte();
    }

    public String next() {
        if (!hasNext())
            throw new NoSuchElementException();
        StringBuilder sb = new StringBuilder();
        int b = readByte();
        while (isPrintableChar(b)) {
            sb.appendCodePoint(b);
            b = readByte();
        }
        return sb.toString();
    }

    public long nextLong() {
        if (!hasNext())
            throw new NoSuchElementException();
        long n = 0;
        boolean minus = false;
        int b = readByte();
        if (b == '-') {
            minus = true;
            b = readByte();
        }
        if (b < '0' || '9' < b) {
            throw new NumberFormatException();
        }
        while (true) {
            if ('0' <= b && b <= '9') {
              n *= 10;
              n += b - '0';
            } else if (b == -1 || !isPrintableChar(b)) {
                return minus ? -n : n;
            } else {
                throw new NumberFormatException();
            }
            b = readByte();
        }
    }

    public int nextInt() {
        return (int) nextLong();
    }

    public long[] nextLongList(int n) {
        return nextLongTable(1, n)[0];
    }

    public int[] nextIntList(int n) {
        return nextIntTable(1, n)[0];
    }

    public long[][] nextLongTable(int n, int m) {
        long[][] ret = new long[n][m];
        for (int i = 0; i < n; i ++) {
            for (int j = 0; j < m; j ++) {
                ret[i][j] = nextLong();
            }
        }
        return ret;
    }

    public int[][] nextIntTable(int n, int m) {
        int[][] ret = new int[n][m];
        for (int i = 0; i < n; i ++) {
            for (int j = 0; j < m; j ++) {
                ret[i][j] = nextInt();
            }
        }
        return ret;
    }
}


Measured memory_bytes: 21588