import sys
readline=sys.stdin.readline
write=sys.stdout.write
from math import gcd as GCD
import math

N=int(readline())
T=list(map(int,readline().split()))
ans_lst=[]
for D in range(12):
    se={(D+i)%12 for i in (0,2,4,5,7,9,11)}
    if all(t in se for t in T):
        ans_lst.append(D)
if len(ans_lst)==1:
    ans=ans_lst[0]
else:
    ans=-1
print(ans)