def f(D):
    return set((D+x)%12 for x in (0,2,4,5,7,9,11))

N=int(input())
T=[int(t) for t in input().split()]
T_set=set(T)

s=set()
for i in range(12):
    if T_set<=f(i):
        s.add(i)
if len(s)==1:
    print(s.pop())
else:
    print(-1)