//#define _GLIBCXX_DEBUG

#include<bits/stdc++.h>
using namespace std;

#define endl '\n'
#define lfs cout<<fixed<<setprecision(10)
#define ALL(a)  (a).begin(),(a).end()
#define ALLR(a)  (a).rbegin(),(a).rend()
#define spa << " " <<
#define fi first
#define se second
#define MP make_pair
#define MT make_tuple
#define PB push_back
#define EB emplace_back
#define rep(i,n,m) for(ll i = (n); i < (ll)(m); i++)
#define rrep(i,n,m) for(ll i = (ll)(m) - 1; i >= (ll)(n); i--)
using ll = long long;
using ld = long double;
const ll MOD1 = 1e9+7;
const ll MOD9 = 998244353;
const ll INF = 1e18;
using P = pair<ll, ll>;
template<typename T1, typename T2>
bool chmin(T1 &a,T2 b){if(a>b){a=b;return true;}else return false;}
template<typename T1, typename T2>
bool chmax(T1 &a,T2 b){if(a<b){a=b;return true;}else return false;}
ll median(ll a,ll b, ll c){return a+b+c-max({a,b,c})-min({a,b,c});}
void ans1(bool x){if(x) cout<<"Yes"<<endl;else cout<<"No"<<endl;}
void ans2(bool x){if(x) cout<<"YES"<<endl;else cout<<"NO"<<endl;}
void ans3(bool x){if(x) cout<<"Yay!"<<endl;else cout<<":("<<endl;}
template<typename T1,typename T2>
void ans(bool x,T1 y,T2 z){if(x)cout<<y<<endl;else cout<<z<<endl;}  
template<typename T>
void debug(vector<vector<T>>&v,ll h,ll w){for(ll i=0;i<h;i++)
{cout<<v[i][0];for(ll j=1;j<w;j++)cout spa v[i][j];cout<<endl;}};
void debug(vector<string>&v,ll h,ll w){for(ll i=0;i<h;i++)
{for(ll j=0;j<w;j++)cout<<v[i][j];cout<<endl;}};
template<typename T>
void debug(vector<T>&v,ll n){if(n!=0)cout<<v[0];
for(ll i=1;i<n;i++)cout spa v[i];cout<<endl;};
template<typename T>
vector<vector<T>>vec(ll x, ll y, T w){
  vector<vector<T>>v(x,vector<T>(y,w));return v;}
ll gcd(ll x,ll y){ll r;while(y!=0&&(r=x%y)!=0){x=y;y=r;}return y==0?x:y;}
vector<ll>dx={1,-1,0,0,1,1,-1,-1};
vector<ll>dy={0,0,1,-1,1,-1,1,-1};
template<typename T>
vector<T> make_v(size_t a,T b){return vector<T>(a,b);}
template<typename... Ts>
auto make_v(size_t a,Ts... ts){
  return vector<decltype(make_v(ts...))>(a,make_v(ts...));
}
template<typename T1, typename T2>
ostream &operator<<(ostream &os, const pair<T1, T2>&p){
  return os << p.first << " " << p.second;
}
//mt19937 mt(chrono::steady_clock::now().time_since_epoch().count());

int main(){
  cin.tie(nullptr);
  ios_base::sync_with_stdio(false);
  ll res=0;
  bool judge = true;
  ll n;cin>>n;
  vector<ll>a(n);
  ll buf1=0,buf2=0;
  auto f=[&](ll x){
    return x&((1<<20)-1);
  };
  rep(i,0,n)cin>>a[i];
  rep(i,2,n){
    buf1^=a[i]&((1<<20)-1);
    buf2^=a[i]>>20;
  }
  res=INF;
  ll a0=a[0]&((1<<20)-1);
  ll a1=a[0]>>20;
  ll b0=a[1]&((1<<20)-1);
  ll b1=a[1]>>20;
  vector<ll>v(4,INF);
  rrep(i,0,1<<20){
    if(a0>=i){
      ll k0=a0-i;
      ll k1=b0+i;
      if(f(k0)^f(k1)^buf1)continue;
      ll idx=0;
      if(k1>=(1<<20))idx|=2;
      v[idx]=i;
    }
    else{
      ll k0=a0-i+(1<<20);
      ll k1=b0+i;
      if(f(k0)^f(k1)^buf1)continue;
      ll idx=1;
      if(k1>=(1<<20))idx|=2;
      v[idx]=i;
    }
  }
  //debug(v,4);
  rep(i,0,1<<20){
    if(a1<i)continue;
    ll k0=a1-i;
    ll k1=b1+i;
    rep(j,0,4){
      ll t0=k0,t1=k1;
      if(j&1)t0-=1;
      if(j&2)t1+=1;
      if(t0<0)continue;
      if(t0^t1^buf2)continue;
      //if(i==0)cout<<t0 spa t1 spa buf2 spa (t0^t1^buf2) spa (i<<20) spa v[j]<<endl;
      chmin(res,(i<<20)+v[j]);
    }
  }
  ans(res<a[0],res,-1);
  return 0;
}

Measured memory_bytes: 3644