//#define NDEBUG
#include <algorithm>
#include <cstddef>
#include <cstdint>
#include <iostream>
#include <utility>
#include <vector>

namespace n91 {

using i8 = std::int_fast8_t;
using i32 = std::int_fast32_t;
using i64 = std::int_fast64_t;
using u8 = std::uint_fast8_t;
using u32 = std::uint_fast32_t;
using u64 = std::uint_fast64_t;
using isize = std::ptrdiff_t;
using usize = std::size_t;

struct rep {
  struct itr {
    usize i;
    constexpr itr(const usize i) noexcept : i(i) {}
    void operator++() noexcept { ++i; }
    constexpr usize operator*() const noexcept { return i; }
    constexpr bool operator!=(const itr x) const noexcept { return i != x.i; }
  };
  const itr f, l;
  constexpr rep(const usize f, const usize l) noexcept
      : f(std::min(f, l)), l(l) {}
  constexpr auto begin() const noexcept { return f; }
  constexpr auto end() const noexcept { return l; }
};
struct revrep {
  struct itr {
    usize i;
    constexpr itr(const usize i) noexcept : i(i) {}
    void operator++() noexcept { --i; }
    constexpr usize operator*() const noexcept { return i; }
    constexpr bool operator!=(const itr x) const noexcept { return i != x.i; }
  };
  const itr f, l;
  constexpr revrep(const usize f, const usize l) noexcept
      : f(l - 1), l(std::min(f, l) - 1) {}
  constexpr auto begin() const noexcept { return f; }
  constexpr auto end() const noexcept { return l; }
};
template <class T> auto md_vec(const usize n, const T &value) {
  return std::vector<T>(n, value);
}
template <class... Args> auto md_vec(const usize n, Args... args) {
  return std::vector<decltype(md_vec(args...))>(n, md_vec(args...));
}
template <class T> constexpr T difference(const T &a, const T &b) noexcept {
  return a < b ? b - a : a - b;
}
template <class T> void chmin(T &a, const T &b) noexcept {
  if (b < a)
    a = b;
}
template <class T> void chmax(T &a, const T &b) noexcept {
  if (a < b)
    a = b;
}
template <class F> class fix_point : private F {
public:
  explicit constexpr fix_point(F &&f) : F(std::forward<F>(f)) {}

  template <class... Args>
  constexpr decltype(auto) operator()(Args &&... args) const {
    return F::operator()(*this, std::forward<Args>(args)...);
  }
};
template <class F> constexpr decltype(auto) make_fix(F &&f) {
  return fix_point<F>(std::forward<F>(f));
}
template <class T> T scan() {
  T ret;
  std::cin >> ret;
  return ret;
}

} // namespace n91
#include <utility>

namespace n91 {

template <class T> constexpr T gcd(T m, T n) noexcept {
  while (n != static_cast<T>(0)) {
    m %= n;
    std::swap(m, n);
  }
  return m;
}

template <class T> constexpr T lcm(const T &m, const T &n) noexcept {
  return m / gcd(m, n) * n;
}

} // namespace n91
#include <algorithm>
#include <utility>
#include <vector>

namespace n91 {

template <class T> std::vector<T> divisors(const T &n) {
  std::vector<T> ret;
  T i = static_cast<T>(1);
  while (i * i < n) {
    if (n % i == static_cast<T>(0)) {
      ret.push_back(i);
      ret.push_back(n / i);
    }
    ++i;
  }
  if (i * i == n) {
    ret.push_back(i);
  }
  std::sort(ret.begin(), ret.end());
  ret.shrink_to_fit();
  return std::move(ret);
}

} // namespace n91
#include <functional>
#include <utility>

namespace n91 {

template <class T, class U, class Operate = std::multiplies<T>>
constexpr T power(T base, U exp, const Operate &oper = Operate(), T iden = 1) {
  while (exp != 0) {
    if (exp % 2 != 0) {
      iden = oper(iden, base);
    }
    exp /= 2;
    base = oper(base, base);
  }
  return iden;
}

} // namespace n91

namespace n91 {

void main_() {
  /*
  std::ios::sync_with_stdio(false);
  std::cin.tie(nullptr);
  //*/
  const usize n = scan<usize>();
  u64 g = 0;
  for (const usize i : rep(0, n)) {
    i64 a;
    std::cin >> a;
    if (a != -1) {
      g = gcd<u64>(g, a);
    }
  }
  if (g == 0) {
    std::cout << "-1";
  } else
    std::cout << g * g << std::endl;
  std::cout << std::endl;
}

} // namespace n91

int main() {
  n91::main_();
  return 0;
}