#define LOCAL
#define _USE_MATH_DEFINES
#include <array>
#include <cassert>
#include <cstdio>
#include <cstring>
#include <iostream>
#include <iomanip>
#include <string>
#include <sstream>
#include <vector>
#include <queue>
#include <stack>
#include <list>
#include <set>
#include <map>
#include <unordered_set>
#include <unordered_map>
#include <algorithm>
#include <complex>
#include <cmath>
#include <numeric>
#include <bitset>
#include <functional>
#include <random>
#include <ctime>

using namespace std;

template <typename A, typename B>
ostream& operator <<(ostream& out, const pair<A, B>& a) {
  out << "(" << a.first << "," << a.second << ")";
  return out;
}
template <typename T, size_t N>
ostream& operator <<(ostream& out, const array<T, N>& a) {
  out << "["; bool first = true;
  for (auto& v : a) { out << (first ? "" : ", "); out << v; first = 0;} out << "]";
  return out;
}
template <typename T>
ostream& operator <<(ostream& out, const vector<T>& a) {
  out << "["; bool first = true;
  for (auto& v : a) { out << (first ? "" : ", "); out << v; first = 0;} out << "]";
  return out;
}
template <typename T, class Cmp>
ostream& operator <<(ostream& out, const set<T, Cmp>& a) {
  out << "{"; bool first = true;
  for (auto& v : a) { out << (first ? "" : ", "); out << v; first = 0;} out << "}";
  return out;
}
template <typename U, typename T, class Cmp>
ostream& operator <<(ostream& out, const map<U, T, Cmp>& a) {
  out << "{"; bool first = true;
  for (auto& p : a) { out << (first ? "" : ", "); out << p.first << ":" << p.second; first = 0;} out << "}";
  return out;
}
#ifdef LOCAL
#define trace(...) __f(#__VA_ARGS__, __VA_ARGS__)
#else
#define trace(...) 42
#endif
template <typename Arg1>
void __f(const char* name, Arg1&& arg1){
  cerr << name << ": " << arg1 << endl;
}
template <typename Arg1, typename... Args>
void __f(const char* names, Arg1&& arg1, Args&&... args){
  const char* comma = strchr(names + 1, ',');
  cerr.write(names, comma - names) << ": " << arg1 << " |";
  __f(comma + 1, args...);
}

typedef long long int64;
typedef pair<int, int> ii;
const int64 INF = 1LL << 60;
const int MOD = 1e9 + 7;
mt19937 mrand(random_device{}());
int rnd(int x) { return mrand() % x; }

struct fast_ios {
  fast_ios() {
    cin.tie(nullptr);
    ios::sync_with_stdio(false);
    cout << fixed << setprecision(10);
  };
} fast_ios_;

const int K = 50;
int x[K], y[K];
int64 sum, xor_sum;
int s[K], t[K];
int64 dp[K][2][2];

int64 solve(int pos, bool carry, bool tight) {
  // trace(pos, carry, tight, x[pos]);
  if (pos < 0) return carry == 0 ? 0 : -INF;
  int64& ret = dp[pos][carry][tight];
  if (ret >= 0) return ret;
  ret = -INF;
  for (int64 i = 0; i < 2; ++i) {
    if (tight && i > x[pos]) continue;
    int j = t[pos] ^ i;
    if (i + j > carry * 2 + s[pos]) continue;
    int nxt_carry = carry * 2 + s[pos] - i - j;
    if (nxt_carry > 1) continue;
    bool nxt_tight = tight && (i == x[pos]);
    // trace(i, j);
    ret = max(ret, (i << pos) + solve(pos - 1, nxt_carry, nxt_tight));
  }
  return ret;
}

int main() {
  int n;
  cin >> n;
  vector<int64> a(n);
  int64 xor_sum = 0;
  for (int i = 0; i < n; ++i) {
    cin >> a[i];
    if (i >= 2) xor_sum ^= a[i];
  }
  int64 sum = a[0] + a[1];
  for (int i = 0; i < K; ++i) {
    x[i] = (a[0] >> i) & 1;
    y[i] = (a[1] >> i) & 1;
    s[i] = (sum >> i) & 1;
    t[i] = (xor_sum >> i) & 1;
  }
  memset(dp, 255, sizeof(dp));
  int64 ret = solve(K - 1, 0, 1);
  trace(a[0], ret);
  if (ret <= 0) {
    cout << -1 << endl;
  } else {
    cout << a[0] - ret << endl;
  }
  return 0;
}


Measured memory_bytes: 3156