N = int(input())
List = list(map(int,input().split()))
gaps = [0,2,4,5,7,9,11]
ans = []
for d in range(12):
    tone = [(d+gap)%12 for gap in gaps]
    flag = 1
    for item in List:
        if item not in tone:
           flag = 0
           break
    if flag == 1:
        ans.append(d)
if len(ans)==1:
    print(ans[0])
else:
    print(-1)