# a,b,c,d = map(int,input().split())
# if a < b and c > d:
#     print("YES")
# else:print("NO")

# n,h = map(int,input().split())
# a = list(map(int,input().split()))
# for i in range(n):
#     a[i] += h
# print(*a)

n = int(input())
t = list(map(int,input().split()))
comb = []
for i in range(12):
    u = []
    u.append(i)
    u.append((i+2)%12)
    u.append((i+4)%12)
    u.append((i+5)%12)
    u.append((i+7)%12)
    u.append((i+9)%12)
    u.append((i+11)%12)
    comb.append(u)
ans = []
for i in range(12):
    tar = comb[i]
    flag = True
    for j in t:
        if j not in tar:
            flag = False
    if flag:
        ans.append(i)
if len(ans) == 1:
    print(ans[0])
else:
    print(-1)