#include<iostream>
#include<string>
#include<cstdio>
#include<vector>
#include<cmath>
#include<algorithm>
#include<functional>
#include<iomanip>
#include<queue>
#include<ciso646>
#include<random>
#include<map>
#include<set>
#include<bitset>
#include<stack>
#include<unordered_map>
#include<utility>
#include<cassert>
#include<complex>
#include<numeric>
using namespace std;

//#define int long long
typedef long long ll;

typedef unsigned long long ul;
typedef unsigned int ui;
constexpr ll mod = 1000000007;
const ll INF = mod * mod;
typedef pair<int, int>P;
#define stop char nyaa;cin>>nyaa;
#define rep(i,n) for(int i=0;i<n;i++)
#define per(i,n) for(int i=n-1;i>=0;i--)
#define Rep(i,sta,n) for(int i=sta;i<n;i++)
#define rep1(i,n) for(int i=1;i<=n;i++)
#define per1(i,n) for(int i=n;i>=1;i--)
#define Rep1(i,sta,n) for(int i=sta;i<=n;i++)
#define all(v) (v).begin(),(v).end()
typedef pair<ll, ll> LP;
typedef long double ld;
typedef pair<ld, ld> LDP;
const ld eps = 1e-12;
const ld pi = acos(-1.0);

ll mod_pow(ll x, ll n) {
	ll res = 1;
	while (n) {
		if (n & 1)res = res * x % mod;
		x = x * x % mod; n >>= 1;
	}
	return res;
}
struct modint {
	ll n;
	modint() :n(0) { ; }
	modint(ll m) :n(m) {
		if (n >= mod)n %= mod;
		else if (n < 0)n = (n % mod + mod) % mod;
	}
	operator int() { return n; }
};
bool operator==(modint a, modint b) { return a.n == b.n; }
modint operator+=(modint& a, modint b) { a.n += b.n; if (a.n >= mod)a.n -= mod; return a; }
modint operator-=(modint& a, modint b) { a.n -= b.n; if (a.n < 0)a.n += mod; return a; }
modint operator*=(modint& a, modint b) { a.n = ((ll)a.n * b.n) % mod; return a; }
modint operator+(modint a, modint b) { return a += b; }
modint operator-(modint a, modint b) { return a -= b; }
modint operator*(modint a, modint b) { return a *= b; }
modint operator^(modint a, int n) {
	if (n == 0)return modint(1);
	modint res = (a * a) ^ (n / 2);
	if (n % 2)res = res * a;
	return res;
}

ll inv(ll a, ll p) {
	return (a == 1 ? 1 : (1 - p * inv(p % a, a)) / a + p);
}
modint operator/(modint a, modint b) { return a * modint(inv(b, mod)); }

const int max_n = 1 << 20;
modint fact[max_n], factinv[max_n];
void init_f() {
	fact[0] = modint(1);
	for (int i = 0; i < max_n - 1; i++) {
		fact[i + 1] = fact[i] * modint(i + 1);
	}
	factinv[max_n - 1] = modint(1) / fact[max_n - 1];
	for (int i = max_n - 2; i >= 0; i--) {
		factinv[i] = factinv[i + 1] * modint(i + 1);
	}
}
modint comb(int a, int b) {
	if (a < 0 || b < 0 || a < b)return 0;
	return fact[a] * factinv[b] * factinv[a - b];
}
modint combP(int a, int b) {
	if (a < 0 || b < 0 || a < b)return 0;
	return fact[a] * factinv[a - b];
}

ll s, sum;
ll inf;

bool used[44][2][2];
ll memo[44][2][2];
ll dp(int t, bool up, bool f) {
	if (t < 0) {
		if (!up) {
			return 0;
		}
		else return INF;
	}
	if (used[t][up][f])return memo[t][up][f];
	used[t][up][f] = true;
	ll res = INF;
	rep(x, 2)rep(y, 2) {
		int c = x ^ y;
		if (s & (1ll << t))c ^= 1;
		if (c)continue;

		if (up) {
			if (x == 0 && y == 0)continue;
			int nt = t - 1;
			bool nup = false;
			if (x == 0 || y == 0)nup = true;
			if (sum & (1ll << t)) {
				if (nup)continue;
				nup = true;
			}
			bool nf = f;
			if (inf & (1ll << t)) {
				if (f && y == 0)continue;
			}
			else {
				if(y==1)nf = false;
			}
			res = min(res, dp(nt, nup, nf) + y*(1ll << t));
		}
		else {
			if (x == 1 && y == 1)continue;
			int nt = t - 1;
			bool nup = false;
			if (sum & (1ll << t)) {
				if (x + y == 0)nup = true;
			}
			else {
				if (x > 0 || y > 0)continue;
			}
			bool nf = f;
			if (inf & (1ll << t)) {
				if (f && y == 0)continue;
			}
			else {
				if(y==1)nf = false;
			}
			res = min(res, dp(nt, nup, nf) + y * (1ll << t));
		}
	}
	return memo[t][up][f] = res;
}

//ll dp[44][2];
void solve() {
	int n; cin >> n;
	vector<ll> a(n);
	rep(i, n)cin >> a[i];
	s = 0;
	Rep(i, 2, n)s ^= a[i];
	sum = a[0] + a[1];
	inf = a[1];
	ll ans = dp(40, false, true);
	//cout << ans << " " << sum << " " << s << "\n";
	if (ans < sum) {
		cout << ans - a[1] << "\n";
	}
	else {
		cout << -1 << "\n";
	}
}


signed main() {
	//ios::sync_with_stdio(false);
	//cin.tie(0);
	//cout << fixed << setprecision(10);
	init_f();
	//int t; cin >> t; rep(i, t)
	solve();
		return 0;
}


Measured memory_bytes: 19980