mod = 1000000007
eps = 10**-9


def main():
    import sys
    input = sys.stdin.readline

    N = int(input())
    T = list(map(int, input().split()))
    A = [0, 2, 4, 5, 7, 9, 11]
    ans = []
    for d in range(12):
        B = [(d+a)%12 for a in A]
        flg = 1
        for t in T:
            if t not in B:
                flg = 0
                break
        if flg:
            ans.append(d)
    if len(ans) == 1:
        print(ans[0])
    else:
        print(-1)


if __name__ == '__main__':
    main()