N = int(input())
T = list(map(int, input().split()))
dx = [0, 2, 4, 5, 7, 9, 11]
ans = []
for d in range(12):
    D = set()
    for x in dx:
        D.add((d+x)%12)
    flag = 1
    for t in T:
        if t not in D:
            flag = 0
            break
    if flag:
        ans.append(d)
if len(ans)==1:
    print(ans[0])
else:
    print(-1)