#include <bits/stdc++.h>
#define F first
#define S second
#define $ ios::sync_with_stdio(0);
#define endl '\n'
using namespace std;
using ll = long long int;
using ii = pair<ll, ll>;
using vi = vector<ll>;
using vii = vector<ii>;
using graph = vector<vi>;
const int INF = 0x3f3f3f3f;
const ll INFL = 0x3f3f3f3f3f3f3f3f;
const int MAXN = 2123;
vector<string> vs;
int main() {$
    int n, m;
    cin >> n >> m;
    string s;
    while (n--) {
        cin >> s;
        vs.emplace_back(s);
    }
    sort(vs.begin(), vs.end());
    for (auto s : vs) {
        cout << s;
    }
    cout << endl;
}