//https://vjudge.net/contest/381915#problem/C
#include <bits/stdc++.h>
using namespace std;

#define el '\n'
#define pb push_back
#define sp ' '
#define all(x) (x).begin(), (x).end()
#define mset(x, y) memset(x, y, sizeof(x))
#define ff first
#define ss second

// clang-format off
bool chmin(int& a, int b){ return b < a ? a = b, true : false; }
bool chmax(int& a, int b){ return b > a ? a = b, true : false; }
bool chmin(long long& a, long long b){ return b < a ? a = b, true : false; }
bool chmax(long long& a, long long b){ return b > a ? a = b, true : false; }
#ifndef ONLINE_JUDGE
template <typename T, size_t N> int SIZE(const T (&t)[N]){ return N; } template<typename T> int SIZE(const T &t){ return t.size(); } string to_string(const string s, int x1=0, int x2=1e9){ return '"' + ((x1 < (int)s.size()) ? s.substr(x1, x2-x1+1) : "") + '"'; } string to_string(const char* s) { return to_string((string) s); } string to_string(const bool b) { return (b ? "true" : "false"); } string to_string(const char c){ return string({c}); } template<size_t N> string to_string(const bitset<N> &b, int x1=0, int x2=1e9){ string t = ""; for(int __iii__ = min(x1,SIZE(b)),  __jjj__ = min(x2, SIZE(b)-1); __iii__ <= __jjj__; ++__iii__){ t += b[__iii__] + '0'; } return '"' + t + '"'; } template <typename A, typename... C> string to_string(const A (&v), int x1=0, int x2=1e9, C... coords); int l_v_l_v_l = 0, t_a_b_s = 0; template <typename A, typename B> string to_string(const pair<A, B> &p) { l_v_l_v_l++; string res = "(" + to_string(p.first) + ", " + to_string(p.second) + ")"; l_v_l_v_l--; return res; } template <typename A, typename... C> string to_string(const A (&v), int x1, int x2, C... coords) { int rnk = rank<A>::value; string tab(t_a_b_s, ' '); string res = ""; bool first = true; if(l_v_l_v_l == 0) res += el; res += tab + "["; x1 = min(x1, SIZE(v)), x2 = min(x2, SIZE(v)); auto l = begin(v); advance(l, x1); auto r = l; advance(r, (x2-x1) + (x2 < SIZE(v))); for (auto e = l; e != r; e = next(e)) { if (!first) { res += ", "; } first = false; l_v_l_v_l++; if(e != l){ if(rnk > 1) { res += el; t_a_b_s = l_v_l_v_l; }; } else{ t_a_b_s = 0; } res += to_string(*e, coords...); l_v_l_v_l--; } res += "]"; if(l_v_l_v_l == 0) res += el; return res; } void dbgm(){;} template<typename Heads, typename... Tails> void dbgm(Heads H, Tails... T){ cout << to_string(H) << " | "; dbgm(T...); } 
#define dbg(...) cout << "[" << #__VA_ARGS__ << "]: "; cout << to_string(__VA_ARGS__) << endl
#define dbgm(...) cout << "[" << #__VA_ARGS__ << "]: "; dbgm(__VA_ARGS__); cout << endl
#else
#define dbg(...) ;
#define dbgm(...) ;
#endif
// clang-format on

typedef long long ll;
typedef unsigned int uint;
typedef unsigned long long ull;
typedef long double ld;
typedef pair<int, int> pii;
typedef pair<pii, pii> ppi;
typedef pair<ll, ll> pll;
typedef pair<int, ll> pil;
typedef vector<int> vi;
typedef vector<ll> vll;
typedef vector<pii> vpii;
typedef vector<vi> vvi;
typedef vector<vll> vvll;
typedef vector<pll> vpll;
typedef vector<vpii> vvpii;
typedef vector<vpll> vvpll;

const int inf = 1e09 + 5e3;
const ll linf = 2e18 + 5e3;
const int mod = 1e9 + 7;
const int mxn = 1e3 + 30;

int n;
ll a[mxn];

int main()
{
    ios_base::sync_with_stdio(0), cin.tie(0), cout.tie(0);

    cin >> n;
    for (int i = 0; i < n; i++)
    {
        cin >> a[i];
    }
    ll zor = 0;
    for (int i = 2; i < n; i++)
        zor ^= a[i];
    ll bf = 0, bs = 0;
    ll sum = a[0] + a[1];
    int f = 1;
    f &= sum >= zor;
    f &= (sum & 1) == (zor & 1);
    bf = bs = (sum - zor) >> 1;
    f &= bf <= a[0];
    for (int i = 60; i >= 0; i--)
    {
        if ((zor & (1ll << i)) == 0)
            continue;
        if (bf + (1ll << i) <= a[0])
        {
            f &= (bf & (1ll << i)) == 0;
            bf |= (1ll << i);
        }
        else
        {
            f &= (bs & (1ll << i)) == 0;
            bs |= (1ll << i);
        }
    }
    // dbgm(bf, bs);
    if (f && bf)
        cout << a[0] - bf;
    else
        cout << -1;
}


Measured memory_bytes: 3664