D=[[d,(d+2)%12,(d+4)%12,(d+5)%12,(d+7)%12,(d+9)%12,(d+11)%12] for d in range(12)]
n=int(input())
l=list(map(int,input().split()))
a=-1

for x in D:
	if sum([i in x for i in l])==n:
		if a==-1: a=x[0]
		else: a=-1; break
print(a)