#include <bits/stdc++.h>

using namespace std;

#define REP(i, n) for (int i=0; i<(n); ++i)
#define RREP(i, n) for (int i=(int)(n)-1; i>=0; --i)
#define FOR(i, a, n) for (int i=(a); i<(n); ++i)
#define RFOR(i, a, n) for (int i=(int)(n)-1; i>=(a); --i)

#define SZ(x) ((int)(x).size())
#define ALL(x) (x).begin(),(x).end()

#define DUMP(x) cerr<<#x<<" = "<<(x)<<endl
#define DEBUG(x) cerr<<#x<<" = "<<(x)<<" (L"<<__LINE__<<")"<<endl;

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 T, class U>
ostream &operator<<(ostream &os, const pair <T, U> &p) {
    return os << "(" << p.first << " " << p.second << ")";
}

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;
}

using ll = long long;
using ull = unsigned long long;
using ld = long double;
using P = pair<int, int>;

using vi = vector<int>;
using vll = vector<ll>;
using vvi = vector<vi>;
using vvll = vector<vll>;

const ll MOD = 1e9 + 7;
const int INF = INT_MAX / 2;
const ll LINF = LLONG_MAX / 2;
const ld eps = 1e-9;


template<int64_t mod>
class modint {
    int64_t x;

public:
    modint(int64_t x = 0) : x(x < 0 ? ((x % mod) + mod) % mod : x % mod) {}

    const modint operator-() const { return x ? mod - x : 0; }

    modint& operator+=(const modint& rhs) {
        if ((x += rhs.x) >= mod) x -= mod;
        return *this;
    }
    modint& operator-=(const modint& rhs) {
        return *this += -rhs;
    }
    modint& operator*=(const modint& rhs) {
        (x *= rhs.x) %= mod;
        return *this;
    }
    modint& operator/=(const modint& rhs) {
        return *this *= rhs.pow(mod - 2);
    }

    friend const modint operator+(modint lhs, const modint& rhs) {
        return lhs += rhs;
    }
    friend const modint operator-(modint lhs, const modint& rhs) {
        return lhs -= rhs;
    }
    friend const modint operator*(modint lhs, const modint& rhs) {
        return lhs *= rhs;
    }
    friend const modint operator/(modint lhs, const modint& rhs) {
        return lhs /= rhs;
    }

    const modint pow(int64_t n) const {
        modint ret = 1, tmp = *this;
        while (n) {
            if (n & 1) ret *= tmp;
            tmp *= tmp; n >>= 1;
        }
        return ret;
    }

    friend bool operator==(const modint& lhs, const modint& rhs) {
        return lhs.x == rhs.x;
    }
    friend bool operator!=(const modint& lhs, const modint& rhs) {
        return !(lhs == rhs);
    }

    friend ostream& operator<<(ostream& os, const modint& a) {
        return os << a.x;
    }
    friend istream& operator>>(istream& is, modint& a) {
        int64_t tmp; is >> tmp; a = tmp;
        return is;
    }
};

template<typename Field>
struct Combination {
    vector<Field> _fact, _rfact, _inv;

    Combination(int n) : _fact(n), _rfact(n), _inv(n) {
        _fact[0] = _rfact[n-1] = 1;
        for (int i = 1; i < n; ++i) _fact[i] = _fact[i-1] * i;
        _rfact[n-1] /= _fact[n-1];
        for (int i = n-1; i > 0; --i) _rfact[i-1] = _rfact[i] * i;
        for (int i = 1; i < n; ++i) _inv[i] = _rfact[i] * _fact[i-1];
    }

    inline Field fact(int k) const { return _fact.at(k); }

    inline Field rfact(int k) const { return _rfact.at(k); }

    inline Field inv(int k) const { assert(k != 0); return _inv.at(k); }

    Field P(int n, int r) const {
        if (r < 0 or n < r) return 0;
        return fact(n) * rfact(n-r);
    }

    Field C(int n, int r) const {
        if (r < 0 or n < r) return 0;
        return fact(n) * rfact(r) * rfact(n-r);
    }

    Field H (int n, int r) const {
        return (n == 0 and r == 0) ? 1 : C(n+r-1, r);
    }
};


int main() {

    int n; cin >> n;
    ll S = 0, X = 0;
    vector<ll> A(n);
    REP(i, n) {
        cin >> A[i];
        if (i < 2) S += A[i];
        else X ^= A[i];
    }

    if (S - X < 0 or (S - X) % 2 == 1) {
        cout << -1 << endl;
        return 0;
    }
    ll D = (S - X) / 2;

    if (D & X) {
        cout << -1 << endl;
        return 0;
    }

    if (D > A[0]) {
        cout << -1 << endl;
        return 0;
    }

    ll a = D;
    RREP(i, 50) {
        if (X >> i & 1) {
            if ((a | 1LL << i) <= A[0]) {
                a |= 1LL << i;
            }
        }
    }

    if (a == 0) {
        cout << -1 << endl;
        return 0;
    }
    ll ans = A[0] - a;
    cout << ans << endl;

    return 0;
}



Measured memory_bytes: 3592