#include <bits/stdc++.h>
using namespace std;
using i64 = long long;
#define rep(i,s,e) for(i64 (i) = (s);(i) < (e);(i)++)
#define all(x) x.begin(),x.end()
#define STRINGIFY(n) #n
#define TOSTRING(n) STRINGIFY(n)
#define PREFIX "#" TOSTRING(__LINE__) "| "
#define debug(x) \
{ \
  std::cout << PREFIX << #x << " = " << x << std::endl; \
}

std::ostream& output_indent(std::ostream& os, int ind) {
  for(int i = 0; i < ind; i++) os << " ";
  return os;
}

template<class S, class T> std::ostream& operator<<(std::ostream& os, const std::pair<S, T>& p);
template<class T> std::ostream& operator<<(std::ostream& os, const std::vector<T>& v);

template<class S, class T> std::ostream& operator<<(std::ostream& os, const std::pair<S, T>& p) {
  return (os << "(" << p.first << ", " << p.second << ")");
}
template<class T> std::ostream& operator<<(std::ostream& os, const std::vector<T>& v) {
  os << "[";
  for(int i = 0;i < v.size();i++) os << v[i] << ", ";
  return (os << "]");
}

template<class T>
static inline std::vector<T> ndvec(size_t&& n, T val) { return std::vector<T>(n, std::forward<T>(val)); }
template<class... Tail>
static inline auto ndvec(size_t&& n, Tail&&... tail) {
  return std::vector<decltype(ndvec(std::forward<Tail>(tail)...))>(n, ndvec(std::forward<Tail>(tail)...));
}
template<class Cond> struct chain {
  Cond cond; chain(Cond cond) : cond(cond) {}
  template<class T> bool operator()(T& a, const T& b) const { if(cond(a, b)) { a = b; return true; } return false; }
};
template<class Cond> chain<Cond> make_chain(Cond cond) { return chain<Cond>(cond); }

#include <iostream>
using i64 = long long;
template<i64 M> struct modint { i64 a;
  constexpr modint(const i64 x = 0): a((x%M+M)%M){}
  constexpr i64 value() const { return a; }
  constexpr modint inv() const { return this->pow(M-2); }
  constexpr modint pow(i64 r) const {
    modint ans(1); modint aa = *this;
    while(r) { if(r & 1) ans *= aa; aa *= aa; r >>= 1; }
    return ans;
  }
  constexpr modint& operator=(const i64 r) { a = (r % M + M) % M; return *this; }
  constexpr modint& operator+=(const modint r) { a += r.a; if(a >= M) a -= M; return *this; }
  constexpr modint& operator-=(const modint r) { a -= r.a; if(a < 0) a += M; return *this; }
  constexpr modint& operator*=(const modint r) { a = a * r.a % M; return *this; }
  constexpr modint& operator/=(const modint r) { (*this) *= r.inv(); return *this; }
  constexpr modint operator+(const modint r) const { return modint(*this) += r; }
  constexpr modint operator-(const modint r) const { return modint(*this) -= r; }
  constexpr modint operator*(const modint r) const { return modint(*this) *= r; }
  constexpr modint operator/(const modint r) const { return modint(*this) /= r; }
  constexpr bool operator!=(const modint r) const { return this->value() != r.value(); }
};

template<const i64 M> std::ostream& operator<<(std::ostream& os, const modint<M>& m) { os << m.value(); return os; }

#include <vector>
#include <tuple>
using i64 = long long;

template<class T>
std::tuple<std::vector<T>, std::vector<T>, std::vector<T>> build_factorial(int N) {
  std::vector<T> fact(N);
  std::vector<T> finv(N);
  std::vector<T> inv(N);
  fact[0] = T(1);
  for(int i = 1;i < N;i++) {
    fact[i] = fact[i - 1] * T(i);
  }
  finv[N - 1] = T(1) / fact[N - 1];
  for(int i = N - 1; i --> 0;) {
    finv[i] = finv[i + 1] * T(i + 1);
  }
  for(int i = 0;i < N;i++) {
    inv[i] = fact[i - 1] * finv[i];
  }
  return std::make_tuple(std::move(fact), std::move(finv), std::move(inv));
}

using fp = modint<(i64)(1e9 + 7)>;

int main() {
  i64 N;
  cin >> N;
  vector<i64> a(N);
  rep(i,0,N) {
    cin >> a[i];
  }
  i64 A = a[0];
  i64 B = a[1];
  i64 X = 0;
  rep(i,2,N) {
    X ^= a[i];
  }
  
  auto dp = ndvec(45, 2, 2, 2, i64(1e18));
  dp[0][0][0][0] = 0;
  rep(i,0,44) {
    rep(f,0,2) {
    rep(d,0,2) {
      rep(u,0,2) {
        i64 ai = (A >> i) & 1;
        i64 bi = (B >> i) & 1;
        i64 x = (X >> i) & 1;
        // 1 引かない
        {
          i64 na = ai ^ d;
          i64 nd = ai == 0 && d == 1;
          i64 nb = bi ^ u;
          i64 nnu = bi == 1 && u == 1;
          i64 nf = f;
          if(ai == 1) nf = 1;
          if((na ^ nb) == x) {
            dp[i + 1][nd][nnu][nf] = std::min(dp[i + 1][nd][nnu][nf], dp[i][d][u][f]);
          }
        }
        {
          i64 na = ai ^ d ^ 1;
          i64 nd = (ai == 0 || d == 1);
          i64 nb = bi ^ u ^ 1;
          i64 nnu = (bi == 1 || u == 1);
          i64 nf = f;
          if((na ^ nb) == x) {
            dp[i + 1][nd][nnu][nf] = std::min(dp[i + 1][nd][nnu][nf], dp[i][d][u][f] + (1LL << i));
          }
        }
      }
    }
    }
  }
  i64 ans = dp[44][0][0][1];
  if(ans == (i64)(1e18)) {
    ans = -1;
  }
  cout << ans << endl;
}


Measured memory_bytes: 3612