n = int(input())

T = list(map(int, input().split()))
T = set(T)
sets = [set() for _ in range(12)]
nums = [0, 2, 4, 5, 7, 9, 11]
for i in range(12):
    for num in nums:
        sets[i].add((num + i) % 12)

kouho = []
for i in range(12):
    if sets[i] & T == T:
        kouho.append(i)

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