import sys
input = sys.stdin.readline
from collections import *

N = int(input())
T = list(map(int, input().split()))
t = set(T)
ans = []

for d in range(12):
    s = set()
    
    for i in [0, 2, 4, 5, 7, 9, 11]:
        s.add((d+i)%12)
    
    if s>=t:
        ans.append(d)

if len(ans)==1:
    print(ans[0])
else:
    print(-1)