#include <algorithm>
#include <bitset>
#include <complex>
#include <deque>
#include <exception>
#include <fstream>
#include <functional>
#include <iomanip>
#include <ios>
#include <iosfwd>
#include <iostream>
#include <istream>
#include <iterator>
#include <limits>
#include <list>
#include <locale>
#include <map>
#include <memory>
#include <new>
#include <numeric>
#include <ostream>
#include <queue>
#include <set>
#include <sstream>
#include <stack>
#include <stdexcept>
#include <streambuf>
#include <string>
#include <typeinfo>
#include <utility>
#include <valarray>
#include <vector>
#include <climits>
#include <cstring>
#include <cassert>


using namespace std;
#define SZ(x) (int)(x.size())
#define REP(i, n) for(int i=0;i<n;++i)
#define FOR(i, a, b) for(int i=a;i<b;++i)
#define RREP(i, n) for(int i=n-1;i>=0;--i)
#define RFOR(i, a, b) for(int i=b-1;i>=a;--i)

#define all(x) begin(x),end(x)
#define dump(x) cerr<<#x<<" = "<<(x)<<endl
#define debug(x) cerr<<#x<<" = "<<(x)<<" (L"<<__LINE__<<")"<< endl;

using ll = long long;
using vi = vector<int>;
using vvi = vector<vi>;
using vll = vector<ll>;
using vvll = vector<vll>;
using P = pair<int, int>;
const double eps = 1e-8;
const ll MOD = 1000000007;
const int INF = INT_MAX / 2;
const ll LINF = LLONG_MAX / 2;

template<class T>
bool chmax(T &a, const T &b) {
    if (a < b) {
        a = b;
        return true;
    }
    return false;
}

template<class T>
bool chmin(T &a, const T &b) {
    if (b < a) {
        a = b;
        return true;
    }
    return false;
}


template<typename T1, typename T2>
ostream &operator<<(ostream &os, const pair<T1, T2> p) {
    os << p.first << ":" << p.second;
    return os;
}

template<class T>
ostream &operator<<(ostream &os, const vector<T> &v) {
    os << "[";
    REP(i, SZ(v)) {
        if (i) os << ", ";
        os << v[i];
    }
    return os << "]";
}

template<class T1, class T2>
ostream &operator<<(ostream &os, const map<T1, T2> &mp) {
    os << "[";
    for (auto it = mp.begin(); it != mp.end(); ++it) {
        if (it != mp.begin()) {
            os << ", ";
        }
        os << *it;
    }
    return os << "]";
}

// edit

int solve(int N, vector<ll> B) {

    ll S = B[0] + B[1], X = 0;
    for (int i = 2; i < N; ++i) X ^= B[i];

    if (((S ^ X) & 1) == 1 || S < X) {
        cout << -1 << endl;
        return -1;
    }

    ll A = (S - X) / 2;
    if (X & A) {
        cout << -1 << endl;
        return -1;
    }

    ll px = X + A;
    ll py = A;

    vector<int> tapu;
    for (int i = 60; i >= 0; --i) {
        if ((px >> i & 1) && !(py >> i & 1)) {
            tapu.push_back(i);
        }
    }

    ll a0 = A;
    for (int i = 0; i < SZ(tapu); ++i) {
        ll tmp = a0 ^(1ll << tapu[i]);
        if (tmp <= B[0]) chmax(a0, tmp);
    }

    if (a0 == 0) {
        cout << -1 << endl;
        return -1;
    }
    if (a0 > B[0]) {
        cout << -1 << endl;
        return -1;
    }

    ll ans = B[0] - a0;
    cout << ans << endl;

    return ans;
}

int greedy(int N, vector<ll> B) {
    ll S = B[0] + B[1], X = 0;
    for (int i = 2; i < N; ++i) X ^= B[i];

    int ans = 0;
    while (B[0] > 0 && (B[0] ^ B[1] ^ X) != 0) {
        B[0]--;
        B[1]++;
        ans++;
    }
    if ((B[0] ^ B[1] ^ X) == 0) {
        return ans;
    }

    return -1;
}


int main() {
    cin.tie(0);
    ios::sync_with_stdio(false);
    cout << fixed << setprecision(10);
//    std::ifstream in("input.txt");
//    std::cin.rdbuf(in.rdbuf());
//    while (true) {
//        int N = rand() % 10 + 2;
//        vector<ll> B;
//        REP(i, N) {
//            ll tmp = rand() % 15 + 1;
//            B.push_back(tmp);
//        }
//
//        int val1 = solve(N, B);
//        int val2 = greedy(N, B);
//        if (val1 != val2) {
//            cout << N << endl;
//            cout << B << endl;
//
//            cout << val1 << " " << val2 << endl;
//            break;
//        }
//
//    }




    int N;
    cin >> N;

    vector<ll> B(N);
    REP(i, N) cin >> B[i];


    solve(N, B);


    return 0;
}


Measured memory_bytes: 3140