n = int(input())
array = list(map(int,input().split()))
count = []
for i in range(0,12):
    flag = True
    for j in array:
        if j in [i,(i+2)%12,(i+4)%12,(i+5)%12,(i+7)%12,(i+9)%12,(i+11)%12]:
            continue
        else:
            flag = False
            break
    if flag:
        count.append(i)
if len(count)==1:
    print(count[0])
else:
    print(-1)