#include <bits/stdc++.h>
using namespace std;
int main()
{
	int n,k;
	cin >> n >> k;
	string s[n];
	string sum[n][n];
	string together[2];
	for (int i = 0; i < n; i++) cin >> s[i];
	for (int i = 0; i < n; i++)
	{
		for (int j = 0; j < n; j++)
		{
			if (s[i].length() + s[i + j].length() == k) sum[i][j] = s[i] + s[i + j];
		}
	}
	int num = 0;
	for (int i = 0; i < n; i++)
	{
		for (int j = 0; j < n; j++)
		{
			for (int k = 0; k < n; k++)
			{
				if (sum[i][j].empty()) continue;
				else if (strcmp(sum[i][j],sum[i][j + k]) < 0) together[num] = sum[i][j];
				else if (strcmp(sum[i][j],sum[i][j + k]) == 0) together[num] = sum[i][j];
				else if (strcmp(sum[i][j],sum[i][j + k]) > 0) together[num] = sum[i][j + k];
			}
			num++;
		}
	}
	cout << together[0];
	return 0;
}