D = [0, 2, 4, 5, 7, 9, 11]

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

C = 0
X = -1
for i in range(12):
    d = set((i+j)%12 for j in D)
    if T <= d:
        C += 1
        X = i

if C == 1:
    print(X)
else:
    print(-1)