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

def make_note(D):
    g = set()
    for a in ad:
        g.add((D+a)%12)
    return g

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

ans = -1
cnt = 0
for d in range(12):
    note = make_note(d)
    if all(T[i] in note for i in range(N)):
        ans = d
        cnt += 1

if cnt == 1:
    print(ans)
else:
    print(-1)