n = int(input())
t = list(map(int, input().split()))
ans = []
mods = {0, 2, 4, 5, 7, 9, 11}
for i in range(12):
	for x in t:
		if (x-i)%12 not in mods:
			break
	else:
		ans.append(i)
if len(ans) == 1:
	print(ans[0])
else:
	print(-1)