n = int(input())
t_list = list(map(int, input().split()))

candidates = []
for D in range(12):
    scale = {(D + x) % 12 for x in [0, 2, 4, 5, 7, 9, 11]}
    valid = True
    for t in t_list:
        if t not in scale:
            valid = False
            break
    if valid:
        candidates.append(D)

if len(candidates) == 1:
    print(candidates[0])
else:
    print(-1)