# import sys; input = sys.stdin.buffer.readline
# sys.setrecursionlimit(10**7)
from collections import defaultdict
con = 10 ** 9 + 7; INF = float("inf")

def getlist():
	return list(map(int, input().split()))

#処理内容
def main():
	N = int(input())
	T = getlist()
	L = [0, 2, 4, 5, 7, 9, 11]
	D = -1
	for d in range(12):
		jud = 1
		for i in T:
			if not ((d + i) % 12 in L):
				jud = 0
				break

		if jud == 1:
			# print(d)
			if D == -1:
				D = (12 - d) % 12
			else:
				print(-1)
				return

	if D == -1:
		print(-1)
	else:
		print(D)

if __name__ == '__main__':
	main()