N = int(input())
lsT = list(map(int,input().split()))
ans = []
for D in range(12):
    set1 = set([D%12, (D+2)%12, (D+4)%12, (D+5)%12, (D+7)%12, (D+9)%12, (D+11)%12])
    f = True
    for i in range(N):
        if lsT[i] in set1:
            continue
        f = False
        break
    if f:
        ans.append(D)
if len(ans) == 1:
    print(ans[0])
else:
    print(-1)