#include <iostream>
#include <string>
#include <map>
#include <algorithm>
#include <list>
#include <vector>
#include <set>
#include <stdio.h>
#include <queue>
#include <stack>
#include <deque>
#include <math.h>
#include <sstream>
#include <stdlib.h>
#include <functional>
using namespace std;

#define rep(i,n) for(int i = 0; i < n; i++)
#define rrep(i,a,n) for(int i = a; i < n; i++)
#define INF (1<<29)
#define INFL 0x3f3f3f3f3f3f3f3fLL
#define MOD 1000000007
#define fi first
#define se second 
#define pb push_back
#define PI 3.14159265358979323846
#define all(o) (o).begin(), (o).end()

typedef double ld;
typedef vector<int> vi;
typedef vector< vi > vvi;
typedef vector<string> vs;
typedef vector<char> vc;
typedef vector<ld> vd;
typedef vector < vc > vvc;
typedef long long ll;
typedef pair<int, int> pii;
typedef unsigned long long int ulli;

//const int dx[] = { 1, 0, -1, 0 };
//const int dy[] = { 0, 1, 0, -1 };

template <typename T> T &chmin(T &a, const T &b) { return a = min(a, b); }
template <typename T> T &chmax(T &a, const T &b) { return a = max(a, b); }

const ld eps = 1e-10, pi = acos(-1.0);

#define MAX_N 100001

// combination
/*
ll fact[MAX_N], factinv[MAX_N];
ll mod_pow(ll n, ll p, ll m) {
	ll a = n;
	ll x = 1;
	while (p) {
		if (p & 1) x = (x * a) % m;
		a = (a * a) % m;
		p >>= 1;
	}
	return x;
}
int extgcd(int a, int b, int& x, int&y) {
	int d = a;
	if (b != 0) {
		d = extgcd(b, a % b, y, x);
		y -= (a / b) * x;
	}
	else {
		x = 1; y = 0;
	}
	return d;
}
int mod_inverse(int a, int m) {
	int x, y;
	extgcd(a, m, x, y);
	return (m + x % m) % m;
}
ll inv(ll n) {
	return mod_pow(n, MOD - 2, MOD);
}
*/
// ダイクストラ
/*
struct edge { int to, cost; };
vector < vector < edge > > G;
vi d;
void dijkstra(int s) {priority_queue<pii, vector< pii >, greater<pii> > que;d[s] = 0;que.push(pii(0, s));while (!que.empty()) {pii p = que.top(); que.pop();int v = p.second;if (d[v] < p.first) continue;rep(i, G[v].size()) {edge e = G[v][i];int cost = e.cost;if (d[v] + cost < d[e.to]) {d[e.to] = d[v] + e.cost;que.push(pii(d[e.to], e.to));}}}}
*/

// Union-Find木
int par[MAX_N], rnk[MAX_N], unionSize[MAX_N];
// はじめは全ての頂点が根
void UnionFindTreeInit(int n) {
	rep(i, n) {
		par[i] = i;
		rnk[i] = 0;
		unionSize[i] = 1;
	}
}
//木の根元を求める
int root(int x) {
	if (par[x] == x) return x; // 根を返す
	else return par[x] = root(par[x]);
}
// 連結成分の大きさを取得
int componentSize(int x) {
	return unionSize[root(x)];
}
// xとyが同じ集合に属するか否か
bool same(int x, int y) {
	return root(x) == root(y);
}
// xとyの属する集合を併合
void unite(int x, int y) {
	x = root(x);
	y = root(y);
	if (x == y) return;
	if (rnk[x] < rnk[y]) {
		par[x] = y;
		unionSize[y] += unionSize[x];
	}
	else {
		par[y] = x;
		if (rnk[x] == rnk[y]) rnk[x]++;
		unionSize[x] += unionSize[y];
	}
}

struct edge {
	int a, b, cost;
	bool operator<(const edge& e) const {
		return cost < e.cost;
	}
};

//-----------------------------------------------------------------

//int dx[8] = { -1, 1, 0, 0, 1, 1, -1, -1 };
//int dy[8] = { 0, 0, 1, -1, 1, -1, -1, 1 };



int main() {
	int n, l;
	cin >> n >> l;
	vs S(n);
	rep(i, n) cin >> S[i];

	list < string > alf[26];

	rep(i, n) alf[S[i][l - 1] - 'a'].push_back(S[i]);
	

	
	for (int i = l - 2; i >= 0; i--) {
		int bucketSize[26] = {};
		rep(j, 26) {
			bucketSize[j] = alf[j].size();
		}
		rep(j, 26) {
			rep(k, bucketSize[j]) {	
				string str = alf[j].front();
				alf[str[i] - 'a'].push_back(str);
				alf[j].pop_front();				
			}
		}
	}
	
	rep(i, 26) {
		while (!alf[i].empty()) {
			cout << alf[i].front();
			alf[i].pop_front();
		}
	}
	cout << endl;
	
	return 0;
}



Measured memory_bytes: 256