import sys
n = int(input())
t_i = [int(i) for i in input().split()]
l = [0, 2, 4, 5, 7, 9, 11]

ans = []
for i in range(12):
    new_l = [(i + j) % 12 for j in l]
    flg = False
    for t in t_i:
        if not t in new_l:
            flg = True
            break
        
    if flg:
        continue
    else:
        ans.append(i)

if len(ans) == 1:
    print(ans[0])
else:
    print(-1)