#include <bits/stdc++.h>
using namespace std;
using lint = long long;
using pint = pair<int, int>;
using plint = pair<lint, lint>;
struct fast_ios { fast_ios(){ cin.tie(nullptr); ios::sync_with_stdio(false); cout << fixed << setprecision(20); }; } fast_ios_;
#define ALL(x) (x).begin(), (x).end()
#define FOR(i, begin, end) for(int i=(begin),i##_end_=(end);i<i##_end_;i++)
#define IFOR(i, begin, end) for(int i=(end)-1,i##_begin_=(begin);i>=i##_begin_;i--)
#define REP(i, n) FOR(i,0,n)
#define IREP(i, n) IFOR(i,0,n)
template <typename T> void ndarray(vector<T> &vec, int len) { vec.resize(len); }
template <typename T, typename... Args> void ndarray(vector<T> &vec, int len, Args... args) { vec.resize(len); for (auto &v : vec) ndarray(v, args...); }
template <typename V, typename T> void ndfill(V &x, const T &val) { x = val; }
template <typename V, typename T> void ndfill(vector<V> &vec, const T &val) { for (auto &v : vec) ndfill(v, val); }
template <typename T> bool chmax(T &m, const T q) { if (m < q) {m = q; return true;} else return false; }
template <typename T> bool chmin(T &m, const T q) { if (m > q) {m = q; return true;} else return false; }
template <typename T1, typename T2> pair<T1, T2> operator+(const pair<T1, T2> &l, const pair<T1, T2> &r) { return make_pair(l.first + r.first, l.second + r.second); }
template <typename T1, typename T2> pair<T1, T2> operator-(const pair<T1, T2> &l, const pair<T1, T2> &r) { return make_pair(l.first - r.first, l.second - r.second); }
template <typename T> vector<T> srtunq(vector<T> vec) { sort(vec.begin(), vec.end()), vec.erase(unique(vec.begin(), vec.end()), vec.end()); return vec; }
template <typename T> istream &operator>>(istream &is, vector<T> &vec) { for (auto &v : vec) is >> v; return is; }
template <typename T> ostream &operator<<(ostream &os, const vector<T> &vec) { os << "["; for (auto v : vec) os << v << ","; os << "]"; return os; }
template <typename T> ostream &operator<<(ostream &os, const deque<T> &vec) { os << "deq["; for (auto v : vec) os << v << ","; os << "]"; return os; }
template <typename T> ostream &operator<<(ostream &os, const set<T> &vec) { os << "{"; for (auto v : vec) os << v << ","; os << "}"; return os; }
template <typename T> ostream &operator<<(ostream &os, const unordered_set<T> &vec) { os << "{"; for (auto v : vec) os << v << ","; os << "}"; return os; }
template <typename T> ostream &operator<<(ostream &os, const multiset<T> &vec) { os << "{"; for (auto v : vec) os << v << ","; os << "}"; return os; }
template <typename T> ostream &operator<<(ostream &os, const unordered_multiset<T> &vec) { os << "{"; for (auto v : vec) os << v << ","; os << "}"; return os; }
template <typename T1, typename T2> ostream &operator<<(ostream &os, const pair<T1, T2> &pa) { os << "(" << pa.first << "," << pa.second << ")"; return os; }
template <typename TK, typename TV> ostream &operator<<(ostream &os, const map<TK, TV> &mp) { os << "{"; for (auto v : mp) os << v.first << "=>" << v.second << ","; os << "}"; return os; }
template <typename TK, typename TV> ostream &operator<<(ostream &os, const unordered_map<TK, TV> &mp) { os << "{"; for (auto v : mp) os << v.first << "=>" << v.second << ","; os << "}"; return os; }
#define dbg(x) cerr << #x << " = " << (x) << " (L" << __LINE__ << ") " << __FILE__ << endl;

void bad()
{
    puts("-1");
    exit(0);
}

int main()
{
    int N;
    cin >> N;
    vector<lint> A(N);
    cin >> A;
    lint tgt = 0, s = A[0] + A[1];
    FOR(i, 2, N) tgt ^= A[i];
    constexpr int D = 42;

    lint x = 0, y = 0;

    lint diff = s - tgt;
    if (s < tgt or diff % 2) bad();
    vector<int> dp(D);
    diff /= 2;
    dbg(tgt);
    dbg(diff);
    REP(d, D) if ((tgt >> d) & 1) dp[d]++;
    REP(d, D) if ((diff >> d) & 1) dp[d] += 2;
    if (*max_element(ALL(dp)) > 2) bad();

    constexpr lint INF = 1e18;
    vector<lint> dsame(D + 1, -INF), dless(D + 1, -INF);
    dsame.back() = 0;
    dbg(s);
    dbg(dp);
    lint S = A[0];

    IREP(d, D)
    {

        REP(p, !!dp[d] + 1)
        {
            if (dp[d] == 2 and p == 0) continue;
            if (p < ((S >> d) & 1)) chmax(dless[d], dsame[d + 1] + p * (1LL << d));
            if (p == ((S >> d) & 1)) chmax(dsame[d], dsame[d + 1] + p * (1LL << d));
            chmax(dless[d], dless[d + 1] + p * (1LL << d));
        }
    }
    dbg(dsame);
    dbg(dless);
    lint ret = max(dsame[0], dless[0]);
    dbg(ret);
    if (ret > 0 and ret <= A[0])
    {
        cout << A[0] - ret << '\n';
    }
    else bad();
}


Measured memory_bytes: 3656