#include <iostream>
#include <vector>
#include <map>

using namespace std;

typedef long long ll;

const int INT_INF = static_cast<const int>(1e9);
const int MOD = static_cast<const int>(1e9 + 7);
const int MOD1 = static_cast<const int>(1e9 + 9);
const int INITI = 1000000;

vector<pair<int, int>> p_pow;

int p = 31;

struct Hash
{
    size_t n;
    vector<pair<int, int>> hash;

    Hash()
    {
        n = 0;
    }

    explicit Hash(string s)
    {
        n = s.size();
        hash.resize(n);
        for (int i = 0; i < n; i++) {
            if (i > 0) {
                hash[i].first = static_cast<int>(1ll * hash[i - 1].first * p % MOD);
            } else {
                hash[i].first = 0;
            }
            hash[i].first += 1ll * (s[i] - 'a' + 1) % MOD;
            hash[i].first %= MOD;
            if (i > 0) {
                hash[i].second = static_cast<int>(1ll * hash[i - 1].second * p % MOD1);
            } else {
                hash[i].second = 0;
            }
            hash[i].second += 1ll * (s[i] - 'a' + 1) % MOD1;
            hash[i].second %= MOD1;

            if (hash[i].first < 0) {
                hash[i].first += MOD;
            }
            if (hash[i].second < 0) {
                hash[i].second += MOD1;
            }
        }
    }

    pair<int, int> get(int l, int r)
    {
        int len = r - l + 1;
        int h1 = hash[r].first;
        if (l != 0) {
            h1 -= 1ll * hash[l - 1].first * p_pow[len].first % MOD;
        }
        h1 %= MOD;
        if (h1 < 0) {
            h1 += MOD;
        }

        int h2 = hash[r].second;
        if (l != 0) {
            h2 -= 1ll * hash[l - 1].second * p_pow[len].second % MOD1;
        }
        h2 %= MOD1;
        if (h2 < 0) {
            h2 += MOD1;
        }
        return make_pair(h1, h2);
    }
};


int main()
{
    ios_base::sync_with_stdio(false);
    cin.tie();
#ifdef LOCAL
    freopen("input.txt", "r", stdin);
    freopen("output.txt", "w", stdout);
#endif
    p_pow.resize(INITI, make_pair(0, 0));
    p_pow[0].first = 1;
    p_pow[0].second = 1;
    for (int i = 1; i < p_pow.size(); i++) {
        p_pow[i].first = static_cast<int>((1ll * p_pow[i - 1].first * p) % MOD);
        p_pow[i].second = static_cast<int>((1ll * p_pow[i - 1].second * p) % MOD1);
    }
    size_t n;
    cin >> n;
    vector<Hash> hash(n);
    vector<string> g(n);
    int mn = INT_INF;
    for (int i = 0; i < n; i++) {
        cin >> g[i];
        if (g[i].size() < mn) {
            mn = g[i].size();
        }
        hash[i] = Hash(g[i]);
    }
    int l = 1, r = mn;
    pair<int, pair<int, int>> ans = {-1, {-1, -1}};
    while (l <= r) {
        int len = (l + r) / 2;
        map<pair<int, int>, pair<int, pair<int, int>>> res;
        for (int k = 0; k < n; k++) {
            map<pair<int, int>, int> prev;
            for (int i = 0; i <= hash[k].n - len; i++) {
                prev[hash[k].get(i, i + len - 1)] = i;
            }
            for (auto& x : prev) {
                res[x.first] = {res[x.first].first + 1, {k, x.second}};
            }
        }
        pair<int, int> cur = {-1, -1};
        for (auto& x : res) {
            if (x.second.first == n) {
                cur = x.second.second;
                break;
            }
        }
        if (cur.first != -1) {
            ans = {len, cur};
            l = len + 1;
        } else {
            r = len - 1;
        }
    }
    cout << g[ans.second.first].substr(static_cast<unsigned int>(ans.second.second),
                                       static_cast<unsigned int>(ans.first));
    return 0;
}