//#pragma GCC optimize("Ofast")
//#pragma GCC optimize("unroll-loops")
//#pragma GCC target("sse,sse2,sse3,ssse3,sse4,popcnt,abm,mmx,avx,tune=native")
#include <bits/stdc++.h>
using namespace std;
using ll = long long;
using ull = unsigned long long;
using db = double;
using ld = long double;
template<typename T> using V = vector<T>;
template<typename T> using VV = vector<vector<T>>;
#define fs first
#define sc second
#define pb push_back
#define mp make_pair
#define mt make_tuple
#define eb emplace_back
#define lb lower_bound
#define ub upper_bound
#define all(v) (v).begin(),(v).end()
#define siz(v) (ll)(v).size()
#define rep(i,a,n) for(ll i=a;i<(ll)(n);++i)
#define repr(i,a,n) for(ll i=n-1;(ll)a<=i;--i)
#define ENDL '\n'
typedef pair<int,int> Pi;
typedef pair<ll,ll> PL;
constexpr ll mod = 1000000007;
constexpr ll INF = 1000000099;
constexpr ll LINF = (ll)(1e18 +99);
const vector<ll> dx={-1,1,0,0},dy={0,0,-1,1};
template<typename T,typename U> inline bool chmin(T& t, const U& u){if(t>u){t=u;return 1;}return 0;}
template<typename T,typename U> inline bool chmax(T& t, const U& u){if(t<u){t=u;return 1;}return 0;}
template<typename T> inline T gcd(T a,T b){return b?gcd(b,a%b):a;}

template<typename T,typename Y> inline T mpow(T a, Y n) {
  T res = 1;
  for(;n;n>>=1) {
    if (n & 1) res = res * a;
    a = a * a;
  }
  return res;
}

template<typename T,typename Y>
ostream& operator<<(ostream& os,const pair<T,Y>& p){
   return os<<"{"<<p.fs<<","<<p.sc<<"}";
}
template<typename T> ostream& operator<<(ostream& os,const V<T>& v){
   os<<"{";
  for(auto e:v)os<<e<<",";
  return os<<"}";
}
template<typename ...Args>
void debug(Args&... args){
  for(auto const& x:{args...}){
    cerr<<x<<' ';
  }
  cerr<<ENDL;
}

ll dp[50][2][2]={};

bool ispop(ll x,int i){
  return (x & (1ll<<i));
}

signed main(){
  cin.tie(0);cerr.tie(0);ios::sync_with_stdio(false);
  cout<<fixed<<setprecision(20);
  ll n;cin>>n;
  ll a,b,tar=0;
  cin>>a>>b;
  rep(i,2,n){
    ll x;cin>>x;
    tar^=x;
  }

  if((a+b-tar)%2==1){
    cout<<-1<<ENDL;
    return 0;
  }


  rep(i,0,50)rep(j,0,2)rep(k,0,2)dp[i][j][k]=LINF;
  dp[0][0][0]=0;

  tar=a+b-tar;
  tar/=2;
  rep(i,0,45){
    rep(j,0,2){
      rep(k,0,2){
        int apop=ispop(a-dp[i][j][k],i);
        int bpop=ispop(b+dp[i][j][k],i);
        int nj=0,nk=0;
        if(apop && j)nj=1;
        if(!bpop && k)nk=1;


//debug(i,j,k);debug(apop,bpop,nj,nk);
        if(ispop(tar,i)){
          if(apop && bpop)chmin(dp[i+1][nj][nk],dp[i][j][k]);
          else if(!apop && !bpop && dp[i][j][k]+(1ll<<i)<a){
            chmin(dp[i+1][1][nk],dp[i][j][k]+(1ll<<i));
          }
        }else{
          if(apop && bpop){
            if(dp[i][j][k]+(1ll<<i)<a)chmin(dp[i+1][nj][1],dp[i][j][k]+(1ll<<i));
          }
          else{
            chmin(dp[i+1][nj][nk],dp[i][j][k]);
            if(apop && dp[i][j][k]+(1ll<<i)<a)chmin(dp[i+1][nj][nk],dp[i][j][k]+(1ll<<i));
            else if (bpop && dp[i][j][k]+(1ll<<i)<a)chmin(dp[i+1][1][1],dp[i][j][k]+(1ll<<i));
          }
        }
      }
    }
  }

  if(dp[45][0][0]>=LINF)cout<<-1<<ENDL;
  else cout<<dp[45][0][0]<<ENDL;
}
//! ( . _ . ) ! 
//CHECK overflow,vector_size,what to output?

Measured memory_bytes: 3564