#include <iostream>
#include <algorithm>
#include <map>
#include <set>
#include <queue>
#include <stack>
#include <numeric>
#include <bitset>
#include <cmath>

static const int MOD = 1000000007;
using ll = long long;
using u32 = unsigned;
using u64 = unsigned long long;
using namespace std;

template<class T> constexpr T INF = ::numeric_limits<T>::max()/32*15+208;

int main() {
    int n;
    cin >> n;
    vector<int> cnt(100);
    for (int i = 0; i < n; ++i) {
        int x;
        scanf("%d", &x);
        cnt[x]++;
    }
    int ans = -1;
    vector<int> a{0, 2, 4, 5, 7, 9, 11};
    for (int i = 0; i < 12; ++i) {
        int c = 0;
        for (auto &&j : a) {
            c += cnt[(i+j)%12];
        }
        if(c == n){
            if(ans != -1){
                puts("-1");
                return 0;
            }else ans = i;
        }
    }
    cout << ans << "\n";
    return 0;
}