import sys

sys.setrecursionlimit(10 ** 6)
stdin = sys.stdin

ni = lambda: int(ns())
na = lambda: list(map(int, stdin.readline().split()))
ns = lambda: stdin.readline().strip()
nsa = lambda: list(stdin.readline().split())
mod = 10 * 7 + 7

n = ni()
T = na()

D = (0, 2, 4, 5, 7, 9, 11)
Ans = []


for i in range(12):
    flag = True
    Di = set(map(lambda x: (x + i) % 12, D))
    for t in T:
        if not(t in Di):
            flag = False
            break
    if flag:
        Ans.append(i)

if len(Ans) == 1:
    print(Ans[0])

else:
    print(-1)