import sys
input = lambda: sys.stdin.readline().strip()

_ = input() # N
T = set(map(int, input().split()))
M = (0,2,4,5,7,9,11)

ans = [
    D for D in range(12)
    if T <= {(D + m) % 12 for m in M}
]

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