N = int(input())
T = list(map(int, input().split()))
t = {0, 2, 4, 5, 7, 9, 11}
X = {i: {(i+j) % 12 for j in t} for i in range(12)}
F = set()
for i in range(12):
    flg = True
    for j in T:
        flg *= (((j - i) % 12) in X[0])
    if flg:
        F.add(i)
if len(F) == 1:
    L = [i for i in F]
    print(L[0])
else:
    print(-1)