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

max_b = max(b_list)

max_bit = 1
while max_b > 1:
    # print(max_b)
    max_bit += 1
    max_b >>= 1

# print(max_bit)

r = 0
for n in range(max_bit):
    k = 2**n
    if b_list[k - 1] == 2**n:
        r += 2**n
    else:
        continue
is_true = True
for n in range(N):
    if (n + 1) & r == b_list[n]:
        continue
    else:
        is_true = False

if is_true:
    print(r)
else:
    print(-1)