#include <iostream>
#include <cstring>
#include <algorithm>
#include <vector>
#include <string>
#include <math.h>
#include <iomanip>
#include <limits>
#include <list>
#include <queue>
#include <tuple>
#include <map>
#include <stack>
#include <set>
#include <bitset>
#include <functional>
#include <cassert>
using namespace std;
#define fast_io ios_base::sync_with_stdio (false) ; cin.tie(0) ; cout.tie(0) ;
#define ll long long int
#define rep(i,n) for(int i=0; i<(int)(n); i++)
#define reps(i,n) for(int i=1; i<=(int)(n); i++)
#define REP(i,n) for(int i=n-1; i>=0; i--)
#define REPS(i,n) for(int i=n; i>0; i--)
#define MOD (long long int)(1e9+7)
#define INF (int)(1e9)
#define LINF (long long int)(1e18)
#define chmax(a, b) a = (((a)<(b)) ? (b) : (a))
#define chmin(a, b) a = (((a)>(b)) ? (b) : (a))
#define all(v) v.begin(), v.end()
typedef pair<int, int> Pii;
typedef pair<ll, ll> Pll;

ll mpow(ll a, ll b){
  if(b==0) return 1;
  else if(b%2==0){ll memo = mpow(a,b/2); return memo*memo%MOD;}
  else return mpow(a,b-1) * a % MOD;
}
ll lpow(ll a, ll b){
  if(b==0) return 1;
  else if(b%2==0){ll memo = lpow(a,b/2); return memo*memo;}
  else return lpow(a,b-1) * a;
}
ll gcd(ll a, ll b){
  if(b==0) return a;
  else return gcd(b, a%b);
}
vector<ll> kaijo_memo;
ll kaijo(ll n){
  if(kaijo_memo.size() > n) return kaijo_memo[n];
  if(kaijo_memo.size() == 0) kaijo_memo.push_back(1);
  while(kaijo_memo.size() <= n) kaijo_memo.push_back(kaijo_memo[kaijo_memo.size()-1] * kaijo_memo.size() % MOD);
  return kaijo_memo[n];
}
vector<ll> gyaku_kaijo_memo;
ll gyaku_kaijo(ll n){
  if(gyaku_kaijo_memo.size() > n) return gyaku_kaijo_memo[n];
  if(gyaku_kaijo_memo.size() == 0) gyaku_kaijo_memo.push_back(1);
  while(gyaku_kaijo_memo.size() <= n) gyaku_kaijo_memo.push_back(gyaku_kaijo_memo[gyaku_kaijo_memo.size()-1] * mpow(gyaku_kaijo_memo.size(), MOD-2) % MOD);
  return gyaku_kaijo_memo[n];
}

ll nCr(ll n, ll r){
  if(n == r) return 1;//0個の丸と-1個の棒みたいな時に時に効く？不安.
  if(n < r || r < 0) return 0;
  ll ret = 1;
  if(n <= 1e7){
    ret *= kaijo(n); ret %= MOD;
    ret *= gyaku_kaijo(r); ret %= MOD;
    ret *= gyaku_kaijo(n-r); ret %= MOD;
  }else{
    rep(i,r){
      ret *= n-i; ret %= MOD;
      ret *= mpow(r-i, MOD-2); ret %= MOD;
    }
  }
  return ret;
}

int main(void){
  fast_io
  cout<<fixed<<setprecision(15);

  ll a1,a2,S;
  ll n;cin>>n;
  cin>>a1>>a2;
  ll a1_ = a1;//, a2_=a2;
  S = 0;
  rep(i,n-2){
    ll inp;cin>>inp;
    S = S^inp;
  }
  ll A = a1+a2;
  a1=0,a2=0;
  A -= S;
  if(A < 0 || A%2 == 1){
    cout<<-1<<endl;
    return 0;
  }
  REPS(i,60){
    if((A&(1ll<<i)) == 0) continue;
    if((S&(1ll<<(i-1))) > 0){
      cout<<-1<<endl;
      return 0;
    }
    a1 += 1ll<<(i-1);
    a2 += 1ll<<(i-1);
  }
  if(a1 > a1_){
    cout<<-1<<endl;
    return 0;
  }
  REP(i,60){
    if((S&(1ll<<i)) == 0){
      continue;
    }
    if(a1 + (1ll<<i) <= a1_){
      a1 += (1ll<<i);
    }else{
      a2 += (1ll<<i);
    }
  }
  if(a1 == 0){
    rep(i,60){
      if((a2 & (1ll<<i)) > 0){
        a1 += (1ll<<i);
        a2 -= (1ll<<i);
        if(a1 > a1_){
          cout<<-1<<endl;
          return 0;
        }
        break;
      }
    }
  }
  if(a1 == 0){
    cout<<-1<<endl;
    return 0;
  }
  cout<<a1_ - a1 <<endl;
  return 0;
}

Measured memory_bytes: 3592