import java.util.Arrays;
import java.util.Scanner;

public class Main {
	public static void main(String[] args) throws Exception {
		Scanner sc = new Scanner(System.in);
		int n = sc.nextInt();
		int[] t = new int[n];
		for (int i = 0; i < n; i++) {
			t[i] = sc.nextInt();
		}
		sc.close();

		int[] d = {0, 2, 4, 5, 7, 9, 11};
		boolean[] match = new boolean[12];
		Arrays.fill(match, true);
		for (int i = 0; i < 12; i++) {
			for (int j = 0; j < n; j++) {
				boolean flg = false;
				for (int j2 = 0; j2 < d.length; j2++) {
					if (t[j] == (i + d[j2]) % 12) {
						flg = true;
						break;
					}
				}
				if (!flg) {
					match[i] = false;
					break;
				}
			}
		}
		int ans = -1;
		for (int i = 0; i < 12; i++) {
			if (match[i]) {
				if (ans == -1) {
					ans = i;
				} else {
					System.out.println(-1);
					return;
				}
			}
		}
		System.out.println(ans);
	}
}