#include <string>
#include <vector>
#include <algorithm>
#include <numeric>
#include <set>
#include <map>
#include <queue>

#include<stack>
#include<bitset>
#include <iostream>
#include <sstream>
#include <cstdio>
#include <cmath>
#include <ctime>
#include <cstring>
#include <cctype>
#include <cassert>
#include <limits>
#include <functional>
#include<unordered_map>

#define rep(i,n) for(int (i)=0;(i)<(int)(n);++(i))

#define reu(i,l,u) for(int (i)=(int)(l);(i)<(int)(u);++(i))
#define aut(r,v) for(auto r:v)

#define each(it,o) for(aut(it, (o).begin()); it != (o).end(); ++ it)
#define all(o) (o).begin(), (o).end()
#define pb(x) push_back(x)
#define pc()  pop_back()

#define ull unsigned long long
#define mp(x,y) make_pair((x),(y))
#define mset(m,v) memset(m,v,sizeof(m))

#define INF 0x3f3f3f3f
#define INFL 0x3f3f3f3f3f3f3f3fLL
using namespace std;
#define endl '\n'

#define st stack<int>



#define vl vector<long long>
#define vi vector<int>
#define vb vector<bool>
#define vc vector<char>
#define pii pair<int,int>
#define vpii vector<pii>
#define vvi vector<vi>
#define vs vector<string>

#define mod 1000000007

#define un  unordered_map<int,int>
#define mii map<int,int>

#define Sort(a) sort(all(a))
#define ED(a) Sort(a), a.erase(unique(all(a)), a.end())//removing all duplicates

#define max3(a, b, c)   max(a, max(b, c))
#define min3(a, b, c)   min(a, min(b, c))
#define Max(a)       *max_element(all(a))
#define Min(a)       *min_element(all(a))
#define MaxP(a)       max_element(all(a)) - a.begin()
#define MinP(a)        min_element(all(a)) - a.begin()

#define allUpper(a)     transform(all(a), a.begin(), :: toupper)
#define allLower(a)     transform(all(a), a.begin(), :: tolower)

#define rev(a)          reverse(all(a))
#define ub(v,k)          upper_bound(all(v), k) - v.begin()
#define lb(v,k)          lower_bound(all(v), k) - v.begin()
#define adv(a,n)        advance(auto it:a,n)
#define RSort(a)         sort(a.rbegin(),a.rend()) //decending order
#define cnt(v,a)             count(all(v),a)
#define bs(v,a)           binary_search(all(v),a)
#define mmax(v)           *max_element(all(v))
#define mmin(v)           *min_element(all(v))
#define popcount(mask)                       __builtin_popcount(mask) // count set bit
#define popcountLL(mask)                     __builtin_popcountll(mask) // for long long
#define X real() // useful for working with #include <complex> for computational geometry
#define Y imag()
#define ll long long
#define ss second
#define ff first

#define trace1(x)                           cerr << #x << ": " << x << endl;
#define trace2(x, y)                        cerr << #x << ": " << x << " | " << #y << ": " << y << endl;
#define trace3(x, y, z)                     cerr << #x << ": " << x << " | " << #y << ": " << y << " | " << #z << ": " << z << endl;
#define trace4(a, b, c, d)                  cerr << #a << ": " << a << " | " << #b << ": " << b << " | " << #c << ": " << c << " | " << #d << ": " << d << endl;
#define trace5(a, b, c, d, e)               cerr << #a << ": " << a << " | " << #b << ": " << b << " | " << #c << ": " << c << " | " << #d << ": " << d << " | " << #e << ": " << e << endl;
#define trace6(a, b, c, d, e, f)            cerr << #a << ": " << a << " | " << #b << ": " << b << " | " << #c << ": " << c << " | " << #d << ": " << d << " | " << #e << ": " << e << " | " << #f << ": " << f << endl;

template <typename T> T gcd(T a, T b) { while (b) b ^= a ^= b ^= a %= b; return a; }
template <typename T> T setbit(T mask, T pos) { return mask |= (1 << pos); }
template <typename T> T resetbit(T mask, T pos) { return mask &= ~(1 << pos); }
template <typename T> T togglebit(T mask, T pos) { return mask ^= (1 << pos); }
template <typename T> T checkbit(T mask, T pos) { return (bool)(mask & (1 << pos)); }
template <typename T> T lcm(T a, T b) { return (a / gcd(a, b)) * b; }



template <typename T> T modu(T a, T b) { return (a < b ? a : a % b); }
template<typename T> T mod_neg(T a, T b) { a = mod(a, b); if (a < 0) { a += b; } return a; }

template <typename T>T expo(T e, T n) { T x = 1, p = e; while (n) { if (n & 1)x = x * p; p = p * p; n >>= 1; } return x; }
template<typename T> T mod_inverse(T a, T n) { T x, y; T d = extended_euclid(a, n, x, y); return (d > 1 ? -1 : mod_neg(x, n)); }

template <typename T>T power(T e, T n, T m) { T x = 1, p = e; while (n) { if (n & 1)x = mod(x * p, m); p = mod(p * p, m); n >>= 1; } return x; }
template <typename T>T powerL(T e, T n, T m) { T x = 1, p = e; while (n) { if (n & 1)x = mulmod(x, p, m); p = mulmod(p, p, m); n >>= 1; } return x; }

bool Pow2(int n) {
    return n && (!(n & (n - 1)));
}
void printc(vc& result) {
    aut(r, result) cout << r << " ";
    cout << endl;
}
void printl(vl& result) {
    aut(r, result) cout << r << " ";
    cout << endl;
}
void print(vi& result) {
    aut(r, result) cout << r << " ";
    cout << endl;
}
// Recursively computes the power
int binpow(int a, int b)
{
    if (b == 0)
        return a;
    int res = binpow(a, b / 2);
    if (b % 2)
        return res * res * a;
    else
        return res * res;
}
// iteratively computes the power
int pow(int a, int b)
{
    int res = 1;
    while (b)
    {
        if (b & 1)
            res = res * a;
        a = a * a;
        b >>= 1;
    }
    return res;
}
int modpow(int a, int b, int m)
{
    int ans = 1;
    while (b)
    {
        if (b & 1)
            ans = (ans * a) % m;
        b /= 2;
        a = (a * a) % m;
    }
    return ans;
}

const int MOD = 1000000000 + 7;
const int MXX = 100000 + 69;
int fact[MXX], invfact[MXX];
int modinv(int k) { return modpow(k, MOD - 2, MOD); }
void combinatorics()
{
    fact[0] = fact[1] = 1;
    for (int i = 2; i < MXX; i++)
    {
        fact[i] = fact[i - 1] * i;
        fact[i] %= MOD;
    }
    invfact[MXX - 1] = modinv(fact[MXX - 1]);
    for (int i = MXX - 2; i >= 0; i--)
    {
        invfact[i] = invfact[i + 1] * (i + 1);
        invfact[i] %= MOD;
    }
}
int nCr(int x, int y)
{
    if (y > x)
        return 0;
    int num = fact[x];
    num *= invfact[y];
    num %= MOD;
    num *= invfact[x - y];
    num %= MOD;
    return num;
}
//ifstream cin("b_read_on.txt"); ofstream cout("output3.txt");
//Use (<<) for multiplication
//Use (>>) for division
//ios_base::sync_with_stdio(false);cin.tie(NULL);cout.tie(NULL);cout<<fixed;cerr.tie(NULL);
// find_by_order -> value at index
// order_of_key -> index of value
// while using (1<<i) use ((ll)1<<(ll)i) 
// in Floyd-Warshall Algo, k is outer loop 
// If an element was not initially in map and if asked mp[a],the element gets inserted 
// a%=mod take a lot of time... try to use it minimum and use memset as it reduces a lot of time usage...use if(a>=mod) a%=mod
//cout<<(double) can be harmful , always use printf(%.9llf)...take scanf("%lf",&p[i][j]) as input , not llf;
//use s.erase(it++) for erasing iterator and then moving to the next one
//never use adj.resize(n) as value is persistent, always erase
//use __builtin_popcountll() for ll
// no of prime numbers in range : (70,19) , (1000,168) , (100000,1229) , (sqrt(10^9),3409) ;
//always check the use of segment tree using bottom-up dp
int dx[] = { 0, 0, 1, -1 };
int dy[] = { 1, -1, 0, 0 }; // 4 Direction
/* int dx[] = {1,-1,0,0,1,1,-1,-1} , dy[] = {0,0,1,-1,1,-1,1,-1}; */ // 8 Direction
/* int dx[] = {1,-1,1,-1,2,2,-2,-2} , dy[] = {2,2,-2,-2,1,-1,1,-1}; */ // Knight Direction
/* int dx[] = {2,-2,1,1,-1,-1} , dy[] = {0,0,1,-1,1,-1}; */ // Hexagonal Direction
int power(int a, int b) {
    int ans = 1;
    while (b) {
        if (b & 1) ans *= a;
        a *= a;
        b >>= 1;
    }return ans;
}
bool isPrime(int n) {
    for (int i = 2; i * i <= n; i++) {
        if (n % i == 0) return false;
    }
    return true;
}

int n, m;

bool yes(int i, int j, vector<vector<char>>& v) {
    int n = v.size();
    int m = v[0].size();
    if (i + 1 < n) {
        if (v[i + 1][j] == 'G') return false;
        else if (v[i + 1][j] == '.') v[i + 1][j] = '#';
    }
    if (j + 1 < m) {
        if (v[i][j + 1] == 'G') return false;
        else if (v[i][j + 1] == '.')v[i][j + 1] = '#';
    }
    if (i - 1 >= 0) {
        if (v[i - 1][j] == 'G') return false;
        else if (v[i - 1][j] == '.') v[i - 1][j] = '#';
    }
    if (j - 1 >= 0) {
        if (v[i][j - 1] == 'G') return false;
        else if (v[i][j - 1] == '.')v[i][j - 1] = '#';
    }
    return true;
}
void dfs(vvi &vis,int x, int y, vector<vc>& v) {
    vis[x][y]=true;
    rep(i, 4) {
        int xx = x + dx[i];
        int yy = y + dy[i];
        if (xx >= 0 && yy >= 0 && xx < n && yy < m && !vis[xx][yy] && (v[xx][yy] == 'G' || v[xx][yy] == '.')) dfs(vis,xx, yy, v);
    }
}






int main() {
    ios_base::sync_with_stdio(false);
    cin.tie(NULL);
    cout.tie(NULL);
    int test;
    test = 1;
   // cin >> test;
    while (test--) {
        int n, l;
        cin >> n >> l;
        vs v(n);
        rep(i, n)cin >> v[i];
        Sort(v);
      
        string ans = "";
        aut(it, v)ans += it;
        cout << ans << endl;
        






    }

}





Measured memory_bytes: 3632