#include <iostream>
#include <string>
#include <vector>
#define rep(i, n) for(i = 0; i < n; i++)
using namespace std;

int n;
string s[1000];
int cnts[1000][26];

signed main() {
	int i, j, k, l;

	cin >> n;
	rep(i, n) cin >> s[i];

	rep(i, n) {
		rep(j, s[i].length()) cnts[i][s[i][j] - 'a']++;
	}

	vector<vector<int>> tiikawa;
	rep(i, n) {
		rep(j, 26) {
			vector<int> myon;
			rep(k, 26) myon.push_back(cnts[i][k]);
			myon[j]++;
			int tima = 0;
			rep(k, n) {
				int cst = 0;
				rep(l, 26) {
					if (cnts[k][l] > myon[l]) break;
					cst += myon[l] - cnts[k][l];
					if (cst >= 2) break;
				}
				if (l == 26) tima++;
				if (tima >= 2) break;
			}
			if (tima == 1) {
				string t = s[i] + (char)('a' + j);
				cout << t << endl;
				return 0;
			}
		}
	}

	cout << -1 << endl;
	return 0;
}