import sys

read = sys.stdin.buffer.read
readline = sys.stdin.buffer.readline
readlines = sys.stdin.buffer.readlines

N, *T = map(int, read().split())

T_se = set(T)
ans = []
for D in range(12):
    D_se = set((D + x) % 12 for x in (0, 2, 4, 5, 7, 9, 11))
    if T_se <= D_se:
        ans.append(D)

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