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

ans = []
for d in range(12):
    temp = [d, d+2, d+4, d+5, d+7, d+9, d+11]
    temp = [s%12 for s in temp]
    temp = set(temp)
    for t in T:
        if t not in temp:
            break
    else:
        ans.append(d)
if len(ans) == 1:
    print(ans[0])
else:
    print(-1)