#include <bits/stdc++.h>

#define all(a) (a).begin(), (a).end()
#define rall(a) (a).rbegin(), (a).rend()
#define rep(i, n) for (ll i = 0, i##_len = (n); i < i##_len; i++)
#define brep(i, a, b) for (ll i = (a), i##_len = (b); i <= i##_len; i++)
#define rrep(i, n) \
  ll i = (n);      \
  ll now = -1;     \
  while (i-- && (now++ || 1))
#define xrep(i, n) for (ll i = 1, i##_len = (n); i <= i##_len; i++)
#define Yes(n) cout << ((n) ? YES : NO) << '\n'
#define rYes(n)                     \
  cout << ((n) ? YES : NO) << '\n'; \
  return
#define Sort(v) sort(all(v))
#define rSort(v) \
  sort(all(v));  \
  reverse(all(v))
#define Unique(v) (v).erase(unique(all(v)), (v).end())
#define eb emplace_back

using namespace std;
using ll = long long;
using vll = vector<ll>;
using vbl = vector<bool>;
using vint = vector<int>;
using vvll = vector<vll>;
using vstr = vector<string>;

// --- functions which take 1 argument ---
static inline void ignore_ret(int x) { x++; }
template <class T>
inline int sgn(T x) {
  return (x > 0) - (x < 0);
}
template <class T>
inline ll factorial(T n) {
  if (n == 0) return 1;
  ll r = 1;
  for (T i = 2; i <= n; i++) r *= i;
  return r;
}
template <class T>
inline map<T, T> factorize(T n) {
  map<T, T> r;
  for (T i = 2; i * i <= n; i++) {
    while (n % i == 0) {
      r[i]++;
      n /= i;
    }
  }
  if (n != 1) r[n] = 1;
  return r;
}

// --- functions which take 2 arguments ---
template <class T>
inline bool chmax(T& a, const T& b) {
  if (a < b) {
    a = b;
    return 1;
  }
  return 0;
}
template <class T>
inline bool chmin(T& a, const T& b) {
  if (a > b) {
    a = b;
    return 1;
  }
  return 0;
}
template <class T>
inline T nPr(T n, T k) {
  if (n < k || n < 0 || k < 0) return 0;
  T r = 1;
  for (T i = n - k + 1; i <= n; i++) r *= i;
  return r;
}
template <class T>
inline T nCr(T n, T k) {
  if (n < k || n < 0 || k < 0) return 0;
  T r = 1;
  k = min(k, n - k);
  for (T i = n - k + 1; i <= n; i++) r *= i;
  return r / factorial(k);
}

// --- functions which take vector(s) as argument(s) ---
template <class T>
inline void print_d(vector<T> v) {
  for (auto x : v) cout << x;
  cout << '\n';
}
template <class T>
inline void print(vector<T> v) {
  cout << "[ ";
  for (auto x : v) cout << x << ' ';
  cout << "]\n";
}
template <class T>
inline T min(vector<T> v) {
  return *min_element(all(v));
}
template <class T>
inline T max(vector<T> v) {
  return *max_element(all(v));
}
template <class T>
inline ll sum(vector<T> v) {
  return reduce(all(v), 0LL);
}
template <class T>
inline T gcd(vector<T> v) {
  T r = v[0];
  for (T i = 1; i < v.size(); i++) r = gcd(r, v[i]);
  return r;
}
template <class T>
inline T lcm(vector<T> v) {
  T r = v[0];
  for (T i = 1; i < v.size(); i++) r = lcm(r, v[i]);
  return r;
}
template <class T>
inline double abs(vector<T> v) {
  return sqrt(reduce(all(v), 0.0, [](int s, int v) { return s += v * v; }));
}
template <class T>
inline T vector_add(T u, T v) {
  ll s = u.size();
  if (s > v.size()) return {};
  T r;
  rep(i, s) r.eb(u[i] + v[i]);
  return r;
}
template <class T>
inline T vector_subtract(T u, T v) {
  ll s = u.size();
  if (s > v.size()) return {};
  T r;
  rep(i, s) r.eb(u[i] - v[i]);
  return r;
}
template <class T>
inline T dot_product(vector<T> u, vector<T> v) {
  if (u.size() < v.size()) return 0;
  T r = 0;
  for (T i = 0; i < u.size(); i++) r += u[i] * v[i];
  return r;
}
template <class T>
inline T cross_product(T u, T v) {
  T r(3);
  r[0] = u[1] * v[2] - u[2] * v[1];
  r[1] = u[2] * v[0] - u[0] * v[2];
  r[2] = u[0] * v[1] - u[1] * v[0];
  return r;
}

// --- Template ends here ---

void solve(ll N, ll L, vector<string> S) {
  Sort(S);
  print_d(S);
}

int main() {
  ll N;
  ignore_ret(scanf("%lld", &N));
  ll L;
  ignore_ret(scanf("%lld", &L));
  vector<string> S(N);
  for (int i = 0; i < N; i++) {
    cin >> S[i];
  }
  solve(N, L, move(S));
  return 0;
}

Measured memory_bytes: 256