import math
import sys
def S(): return sys.stdin.readline().rstrip()
def I(): return int(sys.stdin.readline().rstrip())
def MI(): return map(int, sys.stdin.readline().rstrip().split())
def LI(): return list(map(int, sys.stdin.readline().rstrip().split()))
def LS(): return list(sys.stdin.readline().rstrip().split())

n = I()
t = set(LI())

ans = []
for i in range(1, 13):
    s = set([i%12, (i+2)%12, (i+4)%12, (i+5)%12, (i+7)%12, (i+9)%12, (i+11)%12])
    if s >= t:
        ans.append(i)
if len(ans) != 1:
    print(-1)
else:
    print(ans[0])