# coding: utf-8
# Your code here!

N = int(input())
T = list(map(int,input().split()))

R = (0,2,4,5,7,9,11)

def check(t,d):
    candi = set()
    for r in R:
        candi.add((d + r) % 12)
        
    for t in T:
        if t not in candi:
            return False
    return True
    
ans = None
for D in range(12):
    # その調ですべてのTが当てはまるかチェック
    if check(T,D):
        if ans is None:
            ans = D
        else:
            print(-1)
            exit(0)
if ans is None:
    print(-1)
else:
    print(ans)