#include <bits/stdc++.h>
using namespace std;
using pii = pair<int, int>;
using ll = long long;
#define NMAX 110
#define LMAX 1001

string words[NMAX], sol;
set<string> m;
bool dp[LMAX];

bool exists(int p, int i, int j)
{
	return m.find(words[p].substr(i, j - i + 1)) != m.end();
}

bool ok(int p, int n)
{
	memset(dp, 0, sizeof dp);

	for(int i = 0; i < words[p].size(); ++i)
	{
		for(int j = 0; j <= i && !dp[i]; ++j)
			if(i - j + 1 < words[p].size() && exists(p, j, i) && (j == 0 || dp[j - 1]))
				dp[i] = true;
	}

	return dp[words[p].size() - 1];
}

int main()
{
	#ifndef ONLINE_JUDGE
	freopen("data.in", "r", stdin);
	freopen("data.out", "w", stdout);
	#endif

	int i, n;

	cin >> n;
	for(i = 1; i <= n; ++i)
		cin >> words[i],
		m.insert(words[i]);

	for(i = 1; i <= n; ++i)
	{
		if(ok(i, n) && sol.size() < words[i].size())
			sol = words[i];
	}

	if(sol.empty()) cout << -1 << '\n';
	else cout << sol << '\n';

	return 0;
}