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

MOD = 12
delta = [0, 2, 4, 5, 7, 9, 11]
cand = []
for tyou in range(12):
    flag = True
    s = [(tyou + val) % MOD for val in delta]
    for t in T:
        if t not in s:
            flag = False
            break
    if flag:
        cand.append(tyou)

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