#define MOD_TYPE 1

#pragma region Macros
#include <bits/stdc++.h>
using namespace std;
/*
#include <boost/multiprecision/cpp_int.hpp>
#include <boost/multiprecision/cpp_dec_float.hpp>
using multiInt = boost::multiprecision::cpp_int;
using lld = boost::multiprecision::cpp_dec_float_100;
*/
/*
#pragma GCC target("avx2")
#pragma GCC optimize("O3")
#pragma GCC optimize("unroll-loops")
*/
using ll = long long int;
using ld = long double;
using pii = pair<int, int>;
using pll = pair<ll, ll>;
using pld = pair<ld, ld>;
template <typename Q_type>
using smaller_queue = priority_queue<Q_type, vector<Q_type>, greater<Q_type>>;

constexpr ll MOD = (MOD_TYPE == 1 ? (ll)(1e9 + 7) : 998244353);
//constexpr ll MOD = 1;
constexpr int INF = (int)1e9 + 10;
constexpr ll LINF = (ll)4e18;
constexpr double PI = acos(-1.0);
constexpr double EPS = 1e-11;
constexpr int Dx[] = {0, 0, -1, 1, -1, 1, -1, 1, 0};
constexpr int Dy[] = {1, -1, 0, 0, -1, -1, 1, 1, 0};

#define REP(i, m, n) for (ll i = m; i < (ll)(n); ++i)
#define rep(i, n) REP(i, 0, n)
#define REPI(i, m, n) for (int i = m; i < (int)(n); ++i)
#define repi(i, n) REPI(i, 0, n)
#define MP make_pair
#define MT make_tuple
#define YES(n) cout << ((n) ? "YES" : "NO") << "\n"
#define Yes(n) cout << ((n) ? "Yes" : "No") << "\n"
#define possible(n) cout << ((n) ? "possible" : "impossible") << "\n"
#define Possible(n) cout << ((n) ? "Possible" : "Impossible") << "\n"
#define Yay(n) cout << ((n) ? "Yay!" : ":(") << "\n"
#define all(v) v.begin(), v.end()
#define NP(v) next_permutation(all(v))
#define dbg(x) cerr << #x << ":" << x << "\n";

inline void init_main()
{
  cin.tie(0);
  ios::sync_with_stdio(false);
  cout << setprecision(30) << setiosflags(ios::fixed);
}
template <typename T>
inline bool chmin(T &a, T b)
{
  if (a > b)
  {
    a = b;
    return true;
  }
  return false;
}
template <typename T>
inline bool chmax(T &a, T b)
{
  if (a < b)
  {
    a = b;
    return true;
  }
  return false;
}
inline ll CEIL(ll a, ll b)
{
  return (a + b - 1) / b;
}
template <typename A, size_t N, typename T>
inline void Fill(A (&array)[N], const T &val)
{
  fill((T *)array, (T *)(array + N), val);
}
template <typename T, typename U>
constexpr istream &operator>>(istream &is, pair<T, U> &p) noexcept
{
  is >> p.first >> p.second;
  return is;
}
template <typename T, typename U>
constexpr ostream &operator<<(ostream &os, pair<T, U> &p) noexcept
{
  os << p.first << " " << p.second;
  return os;
}
#pragma endregion

inline bool bit(ll a, int i)
{
  return a & (1LL << i);
}

// a <= u
// a and b = c
// a xor b = x
ll u, c, x = 0;

ll memo[65][2];

ll f(int i, bool less)
{
  if (i == -1)
    return 0;
  if (memo[i][less] != -1)
    return memo[i][less];

  if (bit(x, i))
  {
    if (bit(c, i)) // 不可能
    {
      return memo[i][less] = -LINF;
    }
    else // (0,1) or (1,0)
    {
      ll r1 = -LINF, r2 = -LINF;
      // aが立つ場合
      if (less or bit(u, i))
        r1 = (1LL << i) + f(i - 1, less);
      // aが立たない場合
      if (bit(u, i))
        r2 = f(i - 1, true);
      else
        r2 = f(i - 1, less);
      return memo[i][less] = max(r1, r2);
    }
  }
  else
  {
    if (bit(c, i)) // (1,1)
    {
      // 不可能
      if (!less and !bit(u, i))
        return memo[i][less] = -LINF;
      return memo[i][less] = (1LL << i) + f(i - 1, less);
    }
    else // (0,0)
    {
      if (bit(u, i))
        return memo[i][less] = f(i - 1, true);
      else
        return memo[i][less] = f(i - 1, less);
    }
  }
}

void solve()
{
  Fill(memo, -1);
  int n;
  cin >> n;
  ll a, b;
  cin >> a >> b;
  u = a;
  rep(i, n - 2)
  {
    ll ai;
    cin >> ai;
    x ^= ai;
  }
  ll s = a + b;
  if (abs(s - x) % 2 == 1 or s - x < 0)
  {
    cout << -1 << "\n";
    return;
  }
  c = (s - x) / 2;
  ll res = f(53, false);
  if (res <= 0)
    cout << -1 << "\n";
  else
    cout << a - res << "\n";
}

int main()
{
  init_main();
  solve();
}

Measured memory_bytes: 3616