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

_ = input() # N
T = set(map(int, input().split()))

ans = []
for D in range(12):
    C = (D, D+2, D+4, D+5, D+7, D+9, D+11)
    if T <= {c % 12 for c in C}:
        ans.append(D)

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