#include <iostream>
#include <string>
#include <algorithm>
using namespace std;

int n, l;
string s[105];
int main() {
    cin >> n >> l;
    for (int i = 0; i < n; ++i) cin >> s[i];
    sort(s, s + n, [](string a, string  b) {
            for (int i = 0; i < a.size(); ++i) {
                if (a[i] == b[i]) continue;
                return a[i] < b[i];
            }
            return true;
            });

    for (int i = 0; i < n; ++i) cout << s[i]; 
    return 0;  
}