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