import collections
N = int(input())
T = list(map(int, input().split())) 
CT = collections.Counter(T)
L = [0,2,4,5,7,9,11]
ans = -1
for i in range(12):
    D = []
    for l in L:
        D.append((i+l)%12)
    if not(CT - collections.Counter(D)):
        if ans==-1:
            ans=i
        else:
            print(-1)
            
            exit()
print(ans)