#include <bits/stdc++.h>
#include <algorithm>

#define endl '\n';


const int N = 1e6 + 10;
const int M = 2e6 + 10;
const int P = 998244353; 
const int Base = 3221225477;


int mod(int x)
{
   return x % P;
}


using namespace std;
bool cmp(string a,string b)
{
	return a < b;
}
int n,l;
signed main()
{
	cin >> n >> l;
	string a[n];
	for(int i = 0;i < n;i++)
	{
		cin >> a[i];
	}
	sort(a,a+n,cmp);
	for(int i = 0;i < n;i++)
	{
		cout << a[i];
	}
	
	return 0;
}