#include <bits/stdc++.h>

using namespace std;
 
#define DEBUG(x) cerr<<#x<<": "<<x<<endl;
#define DEBUG_VEC(v) cerr<<#v<<":";for(int i=0;i<v.size();i++) cerr<<" "<<v[i]; cerr<<endl;
#define DEBUG_MAT(v) cerr<<#v<<endl;for(int i=0;i<v.size();i++){for(int j=0;j<v[i].size();j++) {cerr<<v[i][j]<<" ";}cerr<<endl;}
typedef long long ll;
#define int ll
 
#define vi vector<int>
#define vl vector<ll>
#define vii vector< vector<int> >
#define vll vector< vector<ll> >
#define vs vector<string>
#define pii pair<int,int>
#define pis pair<int,string>
#define psi pair<string,int>
#define pll pair<ll,ll>
template<class S, class T> pair<S, T> operator+(const pair<S, T> &s, const pair<S, T> &t) { return pair<S, T>(s.first + t.first, s.second + t.second); }
template<class S, class T> pair<S, T> operator-(const pair<S, T> &s, const pair<S, T> &t) { return pair<S, T>(s.first - t.first, s.second - t.second); }
template<class S, class T> ostream& operator<<(ostream& os, pair<S, T> p) { os << "(" << p.first << ", " << p.second << ")"; return os; }
#define X first
#define Y second
#define rep(i,n) for(int i=0;i<(int)(n);i++)
#define rep1(i,n) for(int i=1;i<=(int)(n);i++)
#define rrep(i,n) for(int i=(int)(n)-1;i>=0;i--)
#define rrep1(i,n) for(int i=(int)(n);i>0;i--)
#define REP(i,a,b) for(int i=a;i<b;i++)
#define in(x, a, b) (a <= x && x < b)
#define all(c) c.begin(),c.end()
template<class T> bool chmax(T &a, const T &b) { if (a<b) { a = b; return 1; } return 0; }
template<class T> bool chmin(T &a, const T &b) { if (a>b) { a = b; return 1; } return 0; }
#define UNIQUE(v) v.erase(std::unique(v.begin(), v.end()), v.end());
const ll inf = 1000000001;
const ll INF = (ll)1e18 + 1;
const long double pi = 3.1415926535897932384626433832795028841971L;
#define Sp(p) cout<<setprecision(25)<< fixed<<p<<endl;
// int dx[4] = {1, 0, -1, 0}, dy[4] = {0, 1, 0, -1};
// int dx2[8] = { 1,1,0,-1,-1,-1,0,1 }, dy2[8] = { 0,1,1,1,0,-1,-1,-1 };
// vi dx = {1, 0, -1, 0}, dy = {0, 1, 0, -1};
// vi dx2 = { 1,1,0,-1,-1,-1,0,1 }, dy2 = { 0,1,1,1,0,-1,-1,-1 };
#define fio() cin.tie(0); ios::sync_with_stdio(false);
const ll MOD = 1000000007;
// const ll MOD = 998244353;
// #define mp make_pair
//#define endl '\n'


signed main() {
    fio();
    
    int n;
    cin >> n;
    vl a(n);
    rep (i, n) cin >> a[i];

    ll x = 0;
    for (int i = 2; i < n; i++) x ^= a[i];
    ll s = a[0] + a[1];

    vector<vll> dp(50, vll(2, vl(2, -INF)));
    dp[0][0][1] = 0;
    rep (i, 49) {
        ll mask = 1LL << i;
        rep (kuri, 2) {
            rep (ika, 2) {
                if (x & mask) {
                    // 01 or 10
                    int ns = (1 + kuri) % 2;
                    if ((bool)(s & mask) != (bool)ns) continue;
                    int nkuri = 0;
                    if (1 + kuri == 2) nkuri = 1;
                    int nika;
                    // 0の時
                    if (a[0] & mask) nika = 1;
                    else nika = ika;
                    chmax(dp[i + 1][nkuri][nika], dp[i][kuri][ika]);
                    // 1の時
                    if (a[0] & mask) nika = ika;
                    else nika = 0;
                    chmax(dp[i + 1][nkuri][nika], dp[i][kuri][ika] + mask);
                }
                else {
                    // 00 or 11
                    int ns = kuri;
                    if ((bool)(s & mask) != (bool)ns) continue;
                    int nkuri, nika;
                    // 00の時
                    nkuri = 0;
                    if (a[0] & mask) nika = 1;
                    else nika = ika;
                    chmax(dp[i + 1][nkuri][nika], dp[i][kuri][ika]);
                    // 11の時
                    nkuri = 1;
                    if (a[0] & mask) nika = ika;
                    else nika = 0;
                    chmax(dp[i + 1][nkuri][nika], dp[i][kuri][ika] + mask);
                }
            }
        }
    }
    // rep (i, 4) {
    //     rep (j, 2) {
    //         rep (k, 2) cout << dp[i][j][k] << " ";
    //         cout << endl;
    //     }
    //     cout << endl;
    // }

    ll ma = dp.back()[0][1];
    if (ma <= 0) {
        cout << -1 << endl;
    }
    else {
        cout << a[0] - ma << endl;
    }
}

Measured memory_bytes: 3672