N = int(input())
T = set(list(map(int, input().split())))

valid = []
for d in range(12):
    memo = set([d, (d+2)%12, (d+4)%12, (d+5)%12, (d+7)%12, (d+9)%12, (d+11)%12])
    for t in T:
        if t not in memo:
            break
    else:
        valid.append(d)

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