// Copyright (C) 2017 Sayutin Dmitry.
//
// This program is free software; you can redistribute it and/or
// modify it under the terms of the GNU General Public License as
// published by the Free Software Foundation; version 3

// This program is distributed in the hope that it will be useful,
// but WITHOUT ANY WARRANTY; without even the implied warranty of
// MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
// GNU General Public License for more details.

// You should have received a copy of the GNU General Public License
// along with this program; If not, see <http://www.gnu.org/licenses/>.

#include <iostream>
#include <vector>
#include <stdint.h>
#include <algorithm>
#include <set>
#include <map>
#include <array>
#include <queue>
#include <stack>
#include <functional>
#include <utility>
#include <string>
#include <assert.h>
#include <iterator>
#include <cstdint>
#include <cinttypes>
#include <string.h>
#include <random>
#include <numeric>
#include <tuple>


using std::cin;
using std::cout;
using std::cerr;

using std::vector;
using std::map;
using std::array;
using std::set;
using std::string;

using std::pair;
using std::make_pair;

using std::min;
using std::abs;
using std::max;

using std::unique;
using std::sort;
using std::generate;
using std::reverse;
using std::min_element;
using std::max_element;

#ifdef LOCAL
#define LASSERT(X) assert(X)
#else
#define LASSERT(X) {}
#endif

template <typename T>
T input() {
    T res;
    cin >> res;
    LASSERT(cin);
    return res;
}

template <typename IT>
void input_seq(IT b, IT e) {
    std::generate(b, e, input<typename std::remove_reference<decltype(*b)>::type>);
}

#define SZ(vec)         int((vec).size())
#define ALL(data)       data.begin(),data.end()
#define RALL(data)      data.rbegin(),data.rend()
#define TYPEMAX(type)   std::numeric_limits<type>::max()
#define TYPEMIN(type)   std::numeric_limits<type>::min()

#define pb push_back
#define eb emplace_back

int solve(vector<int>& a) {
    a[0] += 1;
    if (*std::max_element(ALL(a)) >= 3)
        return 0;
    
    vector<int> sttic;
    int MSK = 0;
    for (int i = 0; i <= 12; ++i)
        if (a[i] == 2) {
            sttic.push_back(i);
            sttic.push_back((24 - i) % 24);
        } else if (a[i] == 1) {
            MSK |= (1 << i);
        }

    int bst = -1;
    for (int i = 0; i != (1 << 13); ++i)
        if ((MSK | i) == MSK) {
            vector<int> lst = sttic;
            
            for (int j = 0; j <= 12; ++j)
                if (a[j] == 1) {
                    if (i & (1 << j))
                        lst.push_back((24 - j) % 24);
                    else
                        lst.push_back(j);
                }

            sort(ALL(lst));
            int res = 1e6;
            
            for (int j = 0; j != SZ(lst); ++j)
                res = min(res, (24 + lst[(j + 1) % SZ(lst)] - lst[j]) % 24);

            bst = max(bst, res);
        }
    
    return bst;
}

int main() {
    std::iostream::sync_with_stdio(false);
    cin.tie(nullptr);
    cout.tie(nullptr);
    
    // code here.
    vector<int> a(13, 0);

    for (int n = input<int>(); n != 0; --n)
        a[input<int>()] += 1;

    cout << solve(a) << "\n";
    
    return 0;
}


Measured memory_bytes: 256