#include <bits/stdc++.h>
using namespace std;
template <typename A, typename B>
string to_string(pair<A, B> p);
template <typename A, typename B, typename C>
string to_string(tuple<A, B, C> p);
template <typename A, typename B, typename C, typename D>
string to_string(tuple<A, B, C, D> p);
string to_string(const string& s) {
  return '"' + s + '"';
}
string to_string(const char* s) {
  return to_string((string) s);
}
string to_string(bool b) {
  return (b ? "true" : "false");
}
string to_string(vector<bool> v) {
  bool first = true;
  string res = "{";
  for (int i = 0; i < static_cast<int>(v.size()); i++) {
    if (!first) {
      res += ", ";
    }
    first = false;
    res += to_string(v[i]);
  }
  res += "}";
  return res;
}
template <size_t N>
string to_string(bitset<N> v) {
  string res = "";
  for (size_t i = 0; i < N; i++) {
    res += static_cast<char>('0' + v[i]);
  }
  return res;
}
template <typename A>
string to_string(A v) {
  bool first = true;
  string res = "{";
  for (const auto &x : v) {
    if (!first) {
      res += ", ";
    }
    first = false;
    res += to_string(x);
  }
  res += "}";
  return res;
}
template <typename A, typename B>
string to_string(pair<A, B> p) {
  return "(" + to_string(p.first) + ", " + to_string(p.second) + ")";
}
template <typename A, typename B, typename C>
string to_string(tuple<A, B, C> p) {
  return "(" + to_string(get<0>(p)) + ", " + to_string(get<1>(p)) + ", " + to_string(get<2>(p)) + ")";
}
template <typename A, typename B, typename C, typename D>
string to_string(tuple<A, B, C, D> p) {
  return "(" + to_string(get<0>(p)) + ", " + to_string(get<1>(p)) + ", " + to_string(get<2>(p)) + ", " + to_string(get<3>(p)) + ")";
}
void debug_out() { cout << endl; }
template <typename Head, typename... Tail>
void debug_out(Head H, Tail... T) {
  cout << " " << to_string(H);
  debug_out(T...);
}
#ifdef LOCAL
#define dbg(...) cout << "[" << #__VA_ARGS__ << "]:", debug_out(__VA_ARGS__)
#else
#define dbg(...) 42
#endif
const long long mod = 1e9+7; // CHECK
long long power(long long a, long long b, long long m = mod) {
    if(b < 0) {
        return power(power(a, -b), mod-2);
    }
    long long x = 1;
    while(b) {
        if(b & 1) {
            x = 1ll * x * a % m;
        }
        a = 1ll * a * a % m;
        b /= 2;
    }
    return x;
}
#define ll long long
const int INT_INF = 1e9+9;
const ll LL_INF = 2e18;
const int N = 3e5+9; // check N, mod

signed main() {  
    ios_base::sync_with_stdio(false); 
    cin.tie(0); cout.tie(0);
    int n; cin >> n;
    vector<ll> a(n);
    ll xr = 0;
    for (int i = 0; i < n; i++) {
        cin >> a[i];
        xr ^= a[i];
    }
    if (xr == 0) {
        cout << "0\n";
        return 0;
    }
    xr = xr ^ a[0] ^ a[1];
    // a & b = (a[0] + a[1] - xr) / 2
    // a ^ b = xr
    // we want a to be as big as possible <= a[0]
    if ((a[0] + a[1] - xr) & 1) {
        cout << "-1\n";
        return 0;
    }
    ll ad = (a[0] + a[1] - xr) / 2;
    const int B = 41;
    ll b = ad, c = ad;
    for (int bit = B-1; bit >= 0; bit--) {
        // 1 0 = 1 0
        // 0 0 = 0 0
        // 1 1 = 0 1
        int u = (xr >> bit & 1);
        int v = (ad >> bit & 1);
        if (u == 1 && v == 1) {
            cout << "-1\n";
            return 0;
        }
        if (u == 0 && v == 0) {

        } else if (u == 0 && v == 1) {
            
        } else if (u == 1 && v == 0) {
            if ((b | (1ll << bit)) > a[0]) {
                c |= (1ll << bit);
            } else {
                b |= (1ll << bit);
            }
        }
    }
    if (b == 0 || b > a[0]) {   
        cout << "-1\n";
        return 0;
    }
    // dbg(b, c, xr, ad);
    cout << (a[0] - b) << '\n';

    




    

    return 0;
}



Measured memory_bytes: 3664