#include <bits/stdc++.h>

using namespace std;

#define FOR(i, a, b) for(int i = a; i < b; i ++)
#define FORS(i, a, b, c) for(int i = a; i < b; i += c)
#define FORN(i, a, b) for(int i = a; i <= b; i ++)
#define FORD(i, a, b) for(int i = a-1; i >= b; i --)
#define FORE(i, b) for(auto &i:b)
#define REP(i, b) FOR(i, 0, b)
#define REPN(i, b) FORN(i, 0, b)
#define SQ(i) (i*i)
#define ALL(a) a.begin(), a.end()
#define ALLA(a,n) a, a+n
#define SORT(a) sort(ALL(a))
#define SORTA(a, n) sort(ALLA(a, n))
#define REV(a) reverse(ALL(a))
#define REVA(a, n) reverse(ALLA(a, n))
#define MAX(a, b) a = max(a, b)
#define MIN(a, b) a = min(a, b)
#define IN(a, b) (a.find(b) != a.end())
#define BACK(a) a.back(); a.RB()
#define QBACK(a) a.top(); a.pop()
#define PRINT(a) FORE(i, a) cout << i << " "; cout << endl

#define PB  push_back
#define RB  pop_back
#define RF  pop_front
#define INS insert
#define F   first
#define S   second
#define UM  unordered_map
#define US  unordered_set
#define PQ  priority_queue

#define IO ios::sync_with_stdio(false); cin.tie(NULL); cout.tie(NULL)
/*#define IO ios::sync_with_stdio(0);*/

typedef long long ll;
typedef unsigned long long ull;

typedef string sg; 
typedef vector<int> vi;
typedef vector<double> vd;
typedef vector<ll> vll;
typedef pair<int,int> pi;
typedef pair<double,double> pd;
typedef pair<ll,ll> pll;
typedef queue<int> qi;
typedef queue<double> qd;
typedef queue<ll> qll;
typedef US<int> si;
typedef US<double> sd;
typedef US<ll> sll;
typedef vector<vi> mi;
typedef vector<vd> md;
typedef vector<vll> mll;
typedef vector<pi> vpi;
typedef vector<pd> vpd;
typedef vector<pll> vpll;

#define sim template < class c
#define ris return * this
#define dor > debug & operator <<
#define eni(x) sim > typename \
enable_if<sizeof dud<c>(0) x 1, debug&>::type operator<<(c i) {
sim > struct rge { c b, e; };
sim > rge<c> range(c i, c j) { return rge<c>{i, j}; }
sim > auto dud(c* x) -> decltype(cerr << *x, 0);
sim > char dud(...);
struct debug {
#ifdef LOCAL
~debug() { cerr << endl; }
eni(!=) cerr << boolalpha << i; ris; }
eni(==) ris << range(begin(i), end(i)); }
sim, class b dor(pair < b, c > d) {
ris << "(" << d.first << ", " << d.second << ")";
}
sim dor(rge<c> d) {
*this << "[";
for (auto it = d.b; it != d.e; ++it)
*this << ", " + 2 * (it == d.b) << *it;
ris << "]";
}
#else
sim dor(const c&) { ris; }
#endif
};
#define imie(...) " [" << #__VA_ARGS__ ": " << (__VA_ARGS__) << "] "
template <typename A, typename B>
string to_string(pair<A, B> p);

template <typename A, typename B, typename C>
string to_string(tuple<A, B, C> p);

template <typename A, typename B, typename C, typename D>
string to_string(tuple<A, B, C, D> p);

string to_string(const string& s) {
return '"' + s + '"';
}

string to_string(const char* s) {
return to_string((string) s);
}

string to_string(bool b) {
return (b ? "true" : "false");
}

string to_string(vector<bool> v) {
bool first = true;
string res = "{";
for (int i = 0; i < static_cast<int>(v.size()); i++) {
if (!first) {
res += ", ";
}
first = false;
res += to_string(v[i]);
}
res += "}";
return res;
}

template <size_t N>
string to_string(bitset<N> v) {
string res = "";
for (size_t i = 0; i < N; i++) {
res += static_cast<char>('0' + v[i]);
}
return res;
}

template <typename A>
string to_string(A v) {
bool first = true;
string res = "{";
for (const auto &x : v) {
if (!first) {
res += ", ";
}
first = false;
res += to_string(x);
}
res += "}";
return res;
}

template <typename A, typename B>
string to_string(pair<A, B> p) {
return "(" + to_string(p.first) + ", " + to_string(p.second) + ")";
}

template <typename A, typename B, typename C>
string to_string(tuple<A, B, C> p) {
return "(" + to_string(get<0>(p)) + ", " + to_string(get<1>(p)) + ", " + to_string(get<2>(p)) + ")";
}

template <typename A, typename B, typename C, typename D>
string to_string(tuple<A, B, C, D> p) {
return "(" + to_string(get<0>(p)) + ", " + to_string(get<1>(p)) + ", " + to_string(get<2>(p)) + ", " + to_string(get<3>(p)) + ")";
}

void debug_out() { cerr << endl; }

template <typename Head, typename... Tail>
void debug_out(Head H, Tail... T) {
cerr << " " << to_string(H);
debug_out(T...);
}

#ifdef LOCAL
#define dbg(...) cerr << "[" << #__VA_ARGS__ << "]:", debug_out(__VA_ARGS__)
#else
#define dbg(...) 42
#endif

int main()
{
	int n, l;
 	scanf("%d%d",&n,&l);
 	vector<sg> v(n);
 	FOR (i,0,n) {
 		sg s; cin >> s;
		v[i] = s;
 		dbg(v);
 	}
 	SORT(v);
 	FOR (i,0,n) {
 		printf("%s",v[i].c_str());
 	}
  	printf("\n");
 	return 0;
}

Measured memory_bytes: 256