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