///////////////////////////////////////////////////////////////////////////////

#include <bits/stdc++.h>
#include <unistd.h>
#include <time.h>
#include <sys/time.h>

using namespace std;

///////////////////////////////////////////////////////////////////////////////

#define DEBUG 0

#define pb push_back
#define V vector
#define M unordered_map
#define S static
#define PI  (3.1415926535)

#define rep(i,n) for(ll i=0LL; i<n; ++i)
#define srep(i,s,n) for(ll i=s; i<n; ++i)
#define rrep(i,n) for(ll i=n-1LL; i>=0LL; --i)
#define ALL(a) (a).begin(),(a).end()
#define RALL(a) (a).rbegin(),(a).rend()
#define CIN(x) do { \
        assert(!cin.eof()); \
        cin >> x; \
        assert(!cin.fail()); \
} while(0);
#if DEBUG
#define debug_print(...) _debug_print(__VA_ARGS__)
#define debug_printf(...) printf(__VA_ARGS__)
#define debug_print_time _debug_print_time
#else // DEBUG
#define debug_print(...)
#define debug_printf(...)
#define debug_print_time
#endif // DEBUG

typedef long long ll;
typedef unsigned long long ull;
typedef tuple<ll, ll> t2;
typedef tuple<ll, ll, ll> t3;
typedef tuple<ll, ll, ll, ll> t4;
typedef tuple<ll, ll, ll, ll, ll> t5;
template <typename T>
using priority_queue_incr = priority_queue<T, V<T>, greater<T> >;

///////////////////////////////////////////////////////////////////////////////

template <typename TT, typename T>
T get_m(M<TT, T> &m, TT k, T default_value)
{
    if (m.find(k) == m.end()) return m[k] = default_value;
    return m[k];
}

template <typename TT, typename T>
void incr_m(M<TT, T> &m, TT k)
{
    if (m.find(k) == m.end()) m[k] = 0;
    m[k]++;
}

void llin(ll &a)
{
    CIN(a);
}

void llinl1(V<ll> &v, ll count)
{
    for (ll i = 0LL; i < count; ++i) {
        ll a;
        CIN(a);
        v.push_back(a);
    }
}

void llinl2(V<t2> &v, ll count)
{
    for (ll i = 0LL; i < count; ++i) {
        ll a, b;
        CIN(a >> b);
        v.push_back(t2(a, b));
    }
}

void llinl3(V<t3> &v, ll count)
{
    for (ll i = 0LL; i < count; ++i) {
        ll a, b, c;
        CIN(a >> b >> c);
        v.push_back(t3(a, b, c));
    }
}

void llinl4(V<t4> &v, ll count)
{
    for (ll i = 0LL; i < count; ++i) {
        ll a, b, c, d;
        CIN(a >> b >> c >> d);
        v.push_back(t4(a, b, c, d));
    }
}

void llina(V<ll> &v, ll count)
{
    llinl1(v, count);
}

template <typename T>
void sort(V<T> &v)
{
    sort(v.begin(), v.end());
}

template <typename T>
void sort_reverse(V<T> &v)
{
    sort(v.begin(), v.end(), greater<T>());
}

template <typename T>
void _debug_print(T x)
{
    cout << x << endl;
}

void _debug_print(const t2 &x)
{
    ll x1 = get<0>(x);
    ll x2 = get<1>(x);
    cout << "-- " << x1 << " -- " << x2 << endl;
}

void _debug_print(const t3 &x)
{
    ll x1 = get<0>(x);
    ll x2 = get<1>(x);
    ll x3 = get<2>(x);
    cout << "-- " << x1 << " -- " << x2 << " -- " << x3 << endl;
}

void _debug_print(const t4 &x)
{
    ll x1 = get<0>(x);
    ll x2 = get<1>(x);
    ll x3 = get<2>(x);
    ll x4 = get<3>(x);
    cout << "-- " << x1 << " -- " << x2
         << " -- " << x3 << " -- " << x4 << endl;
}

template <typename T>
void _debug_print(T xarray[], ll n)
{
    rep (i, n) _debug_print(xarray[i]);
}

template <typename T>
void _debug_print(const V<T> &xlist)
{
    for (auto x : xlist) {
        cout << "-- ";
        _debug_print(x);
    }
}

template <typename T>
void _debug_print(const set<T> &xset)
{
    for (auto x : xset) {
        cout << "-- ";
        _debug_print(x);
    }
}

template <typename TT, typename T>
void _debug_print(const M<TT, T> &xlist)
{
    for (auto x : xlist) {
        TT k = x.first;
        T v = x.second;
        cout << "====" << endl;
        cout << "K=";
        _debug_print(k);
        cout << "V=";
        _debug_print(v);
    }
}

void _debug_print_time(const char *prefix)
{
    struct timeval tv;
    gettimeofday(&tv, NULL);
    struct tm *tm = localtime(&tv.tv_sec);
    printf("-- %s %02d:%02d:%02d.%06ld\n",
           prefix, tm->tm_hour, tm->tm_min, tm->tm_sec, tv.tv_usec);
}

///////////////////////////////////////////////////////////////////////////////

void _main();
int main()
{
    cout << setprecision(12);
#if !DEBUG
    ios::sync_with_stdio(false);
    cin.tie(0);
#endif

    _main();
    return 0;
}

ll comp(ll val, ll mask)
{
    ll pos = 0;
    ll ret = 0;
    while (mask) {
        ll bit = (mask & -mask);
        mask &= ~bit;
        if (val & bit) ret |= (1LL<<pos);
        pos++;
    }
    return ret;
}

ll decomp(ll val, ll mask)
{
    ll pos = 0;
    ll ret = 0;
    while (mask) {
        ll bit = (mask & -mask);
        mask &= ~bit;
        if (val & (1LL<<pos)) ret |= bit;
        pos++;
    }
    return ret;
}

void _main()
{
    ll n; llin(n);
    V<ll> alist; llina(alist, n);

    ll aim = 0;
    srep (i, 2, n) aim ^= alist[i];

    debug_printf("---- aim = %lld\n", aim);

    ll sum = alist[0] + alist[1];
    ll diff = sum - aim;

    if (diff & 1LL) {
        cout << -1 << endl;
        return;
    }

    ll both_bits = (diff>>1LL);
    debug_printf("sum=%lld diff=%lld both_bits=%lld\n", sum, diff, both_bits);

    while (both_bits) {
        ll bit = (both_bits & -both_bits);
        if (aim & bit) {
            cout << -1 << endl;
            return;
        }
        debug_printf("xx1 %lld %lld\n", both_bits, bit);
        both_bits &= ~bit;
        debug_printf("xx2 %lld %lld\n", both_bits, bit);
    }

    both_bits = (diff>>1LL);

    if (both_bits > alist[0]) {
        cout << -1 << endl;
        return;
    }
    if (both_bits + aim < alist[1]) {
        cout << -1 << endl;
        return;
    }

    ll maxcomp = comp(aim, aim);
    ll left = 0;
    ll right = maxcomp;
    ll now = (left + right) / 2LL;
    ll res = -1e18;
    for ( ; left <= right; now = (left + right) / 2LL) {
        ll aimbit0 = decomp(now, aim);
        ll aimbit1 = decomp(maxcomp-now, aim);
        debug_printf("aim=%lld now=%lld aimbit0=%lld aimbit1=%lld\n",
                     aim, now, aimbit0, aimbit1);
        ll a0 = aimbit0 + both_bits;
        ll a1 = aimbit1 + both_bits;
        if (a0 <= alist[0] && a1 >= alist[1]) {
            res = max(res, now);
            left = now + 1LL;
        }
        else {
            right = now - 1LL;
        }
    }

    ll aimbit_for_a0 = decomp(res, aim);
    debug_printf("aimbit_for_a0 = %lld\n", aimbit_for_a0);
    ll new_a0 = aimbit_for_a0 + both_bits;

    if (!new_a0) {
        cout << -1 << endl;
        return;
    }
    if (new_a0 > alist[0]) {
        cout << -1 << endl;
        return;
    }

    debug_printf("---- ans\n");
    cout << alist[0] - new_a0 << endl;
}

///////////////////////////////////////////////////////////////////////////////


Measured memory_bytes: 3644