# https://yukicoder.me/problems/no/3249

MOD = 10 ** 9 + 7

def main():
    N = int(input())
    B = list(map(int, input().split()))

    n = N
    k = 0
    while n > 0:
        k += 1
        n //= 2
    candidate = 0
    for i in range(k):
        b = B[(1 << i) - 1]
        o = 1 if b & (1 << i) > 0 else 0
        candidate += o * (1 << i)

    for i in range(N):
        if B[i] != ((i + 1) & candidate):
            print(-1)
            return
    if candidate == 0:
        print(-1)
    else:
        print(candidate)



if __name__ == "__main__":
    main()