#include <bits/stdc++.h>

[[nodiscard]] static inline constexpr int_fast32_t solve(const uint_fast32_t n, const std::vector<uint_least32_t>& b) noexcept
{
	uint_fast32_t ans = 0;
	for (uint_fast32_t i = 1; i <= n; ++i)
		for (uint_fast32_t j = 0; j != 18; ++j)
			if ((b[i] >> j) & 1)
			{
				if (!((i >> j) & 1)) [[unlikely]] return -1;
				else ans |= UINT32_C(1) << j;
			}

	if (ans == 0)
		for (ans = 1; ans <= n; ans <<= 1);

	return ans;
}

int main()
{
	std::cin.tie(nullptr);
	std::ios::sync_with_stdio(false);
	
	uint_fast32_t n, i;
	std::cin >> n;
	std::vector<uint_least32_t> b(n + 1);
	for (i = 1; i <= n; ++i)
		std::cin >> b[i];

	std::cout << solve(n, b) << '\n';
	return 0;
}