import java.util.*;

public class Main {
    public static void main(String[] args) {
        Scanner sc = new Scanner(System.in);
        int n = sc.nextInt();
        HashSet<Integer> correct = new HashSet<>();
        correct.add(0);
        correct.add(2);
        correct.add(4);
        correct.add(5);
        correct.add(7);
        correct.add(9);
        correct.add(11);
        boolean[] durs = new boolean[12];
        Arrays.fill(durs, true);
        for (int i = 0; i < n; i++) {
            int x = sc.nextInt();
            for (int j = 0; j < 12; j++) {
                if (!durs[j]) {
                    continue;
                }
                if (!correct.contains((x - j + 12) % 12)) {
                    durs[j] = false;
                }
            }
        }
        int ans = -1;
        for (int i = 0; i < 12; i++) {
            if (durs[i]) {
                if (ans == -1) {
                    ans = i;
                } else {
                    ans = -1;
                    break;
                }
            }
        }
        System.out.println(ans);
    }
}