#include<bits/stdc++.h>
#define rep(i, n) for(ll i = 0; i < (ll)(n); i++)
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 (b<a) { a=b; return 1; } return 0; }
#define all(x) (x).begin(),(x).end()
using namespace std;
typedef long long int ll;
typedef long double ld;
const ll INF=(1LL<<62);
const ld pi=acosl((ld)-1);
const ll mod = 1000000007;
// const ll mod = 1234567;
const int dx[4]={0,1,0,-1};
const int dy[4]={1,0,-1,0};
const int ddx[8]={1,0,-1,-1,-1,0,1,1};
const int ddy[8]={1,1,1,0,-1,-1,-1,0};
#define endn "\n"
#define TO_STRING(VariableName) # VariableName

template <typename T>ostream &operator<<(ostream &out,const vector<T> &v) {rep(i,(int)v.size()-1)cout<<v[i]<<" ";cout<<v[(int)v.size()-1];return out;}
template <typename T1, typename T2>ostream &operator<<(ostream &out, const map<T1, T2> &p) {out << "(" << p.first << ", " << p.second << ")";return out;}
template <typename T1, typename T2>ostream &operator<<(ostream &out, const pair<T1, T2> &p){out << "(" << p.first << ", " << p.second << ")";return out;}
template<class T>void debag(const T &obj){cout<<obj<<endl;}

template< int mod >
struct ModInt {
  int x;

  ModInt() : x(0) {}

  ModInt(int64_t y) : x(y >= 0 ? y % mod : (mod - (-y) % mod) % mod) {}

  ModInt &operator+=(const ModInt &p) {
    if((x += p.x) >= mod) x -= mod;
    return *this;
  }

  ModInt &operator-=(const ModInt &p) {
    if((x += mod - p.x) >= mod) x -= mod;
    return *this;
  }

  ModInt &operator*=(const ModInt &p) {
    x = (int) (1LL * x * p.x % mod);
    return *this;
  }

  ModInt &operator/=(const ModInt &p) {
    *this *= p.inverse();
    return *this;
  }

  ModInt operator-() const { return ModInt(-x); }

  ModInt operator+(const ModInt &p) const { return ModInt(*this) += p; }

  ModInt operator-(const ModInt &p) const { return ModInt(*this) -= p; }

  ModInt operator*(const ModInt &p) const { return ModInt(*this) *= p; }

  ModInt operator/(const ModInt &p) const { return ModInt(*this) /= p; }

  bool operator==(const ModInt &p) const { return x == p.x; }

  bool operator!=(const ModInt &p) const { return x != p.x; }

  ModInt inverse() const {
    int a = x, b = mod, u = 1, v = 0, t;
    while(b > 0) {
      t = a / b;
      swap(a -= t * b, b);
      swap(u -= t * v, v);
    }
    return ModInt(u);
  }

  ModInt pow(int64_t n) const {
    ModInt ret(1), mul(x);
    while(n > 0) {
      if(n & 1) ret *= mul;
      mul *= mul;
      n >>= 1;
    }
    return ret;
  }

  friend ostream &operator<<(ostream &os, const ModInt &p) {
    return os << p.x;
  }

  friend istream &operator>>(istream &is, ModInt &a) {
    int64_t t;
    is >> t;
    a = ModInt< mod >(t);
    return (is);
  }

  static int get_mod() { return mod; }
};

using modint = ModInt< mod >;

template< typename T >
struct Combination {
  vector< T > _fact, _rfact, _inv;

  Combination(int sz) : _fact(sz + 1), _rfact(sz + 1), _inv(sz + 1) {
    _fact[0] = _rfact[sz] = _inv[0] = 1;
    for(int i = 1; i <= sz; i++) _fact[i] = _fact[i - 1] * i;
    _rfact[sz] /= _fact[sz];
    for(int i = sz - 1; i >= 0; i--) _rfact[i] = _rfact[i + 1] * (i + 1);
    for(int i = 1; i <= sz; i++) _inv[i] = _rfact[i] * _fact[i - 1];
  }

  inline T fact(int k) const { return _fact[k]; }

  inline T rfact(int k) const { return _rfact[k]; }

  inline T inv(int k) const { return _inv[k]; }

  T P(int n, int r) const {
    if(r < 0 || n < r) return 0;
    return fact(n) * rfact(n - r);
  }

  T C(int p, int q) const {
    if(q < 0 || p < q) return 0;
    return fact(p) * rfact(q) * rfact(p - q);
  }

  T H(int n, int r) const {
    if(n < 0 || r < 0) return (0);
    return r == 0 ? 1 : C(n + r - 1, r);
  }
};

bool solve(){
  ll n;cin>>n;
  vector<ll>a(n);
  ll sum=0;
  rep(i,n)cin>>a[i];
  for(int i=2;i<n;i++)sum^=a[i];
  ll X=sum^0;
  ll num=0;
  auto getdigit=[](ll x,ll i){
    return ((x>>i)&1);
  };
  ll Y=a[0]+a[1];
  ll A=0;
  ll B=0;
  vector<ll>v;
  for(ll i=61;i>=0;i--){
    //決める
    if(getdigit(X,i)==1){
      B+=(ll)pow(2LL,i);
      v.push_back(i);
    }else{
      if(A+B+2*pow(2LL,i)>Y);
      else{
        A+=pow(2LL,i);
        B+=pow(2LL,i);
      }
    }
    // cout<<i<<" "<<A<<" "<<B<<endl;
    if(A+B>Y)break;
  }
  if(A+B!=Y){
    cout<<-1<<endl;
    return false;
  }
  for(auto&& i:v){
    if(A+pow(2,i)<a[0])A+=pow(2,i);
  }
  if(A==0 or A>a[0]){
    cout<<-1<<endl;
    return false;
  }
  cout<<a[0]-A<<endl;
  // cout<<(A+B)


  return false;
}

signed main(){
  ios::sync_with_stdio(false);
  cin.tie(nullptr);
  cout<<fixed<<setprecision(30);
  solve();
}


Measured memory_bytes: 3964