#include <bits/stdc++.h>
using namespace std;
typedef long long ll;
const ll INF = 1e18;
#define rep(i, a, b) for (int i = a; i < b; i++)
#define per(i, a, b) for (int i = b - 1; i >= a; i--)
#define int long long
using pint = pair<int, int>;
int dx[4] = {1, 0, -1, 0}, dy[4] = {0, 1, 0, -1};
int dxx[8] = {1, 1, 1, 0, 0, -1, -1, -1}, dyy[8] = {-1, 0, 1, -1, 1, -1, 0, 1};

signed main() {
    int n;
    cin >> n;
    int t[n];
    rep (i, 0, n) cin >> t[i];
    vector<int> vec;
    rep (i, 0, n) {
        bool chk = 1;
        rep (j, 0, n) {
            if (i == j) continue;
            int s = (t[j] - t[i]) % 12;
            if (s < 0) s += 12;
            if (s == 2 || s == 4 || s == 5 || s == 7 || s == 9 || s == 11) {
                continue;
            } else {
                chk = 0;
                break;
            }
        }
        if (chk) vec.push_back (t[i]);
    }
    if (vec.size() == 1) {
        cout << vec[0] << "\n";
    } else {
        cout << "-1\n";
    }
}