n = int(input())
T = list(map(int,input().split()))
ans = -1

def check(x):
    for t in T:
        ok = 0
        for j in ((0,2,4,5,7,9,11)):
            if t == (x+j)%12:
                ok = 1
        if ok == 0:
            return False
    return True
for i in range(12):
    if check(i):
        if ans != -1:
            print(-1)
            exit()
        ans = i
print(ans)