#include <bits/stdc++.h>
using namespace std;
#pragma region Macros
typedef long long ll;
#define FOR(i,a,b) for (int i=(a);i<(b);++i)
#define REP(i,n) FOR(i,0,n)
#define ALL(x) (x).begin(),(x).end()
const long long MOD=1e9+7;
// const long long MOD=998244353;
const int INF=1e9;
const long long IINF=1e18;
const int dx[4]={1,0,-1,0},dy[4]={0,1,0,-1};
const char dir[4]={'D','R','U','L'};
#define LOCAL

template<typename T>
istream &operator>>(istream &is,vector<T> &v){
    for (T &x:v) is >> x;
    return is;
}
template<typename T>
ostream &operator<<(ostream &os,const vector<T> &v){
    for (int i=0;i<v.size();++i){
        os << v[i] << (i+1==v.size()?"": " ");
    }
    return os;
}
template<typename T,typename U>
ostream &operator<<(ostream &os,const pair<T,U> &p){
    cout << '(' << p.first << ',' << p.second << ')';
    return os;
}
template<typename T,typename U>
ostream &operator<<(ostream &os,const map<T,U> &m){
    os << '{';
    for (auto itr=m.begin();itr!=m.end();++itr){
        os << '(' << itr->first << ',' << itr->second << ')';
        if (++itr!=m.end()) os << ',';
        --itr;
    }
    os << '}';
    return os;
}
template<typename T>
ostream &operator<<(ostream &os,const set<T> &s){
    os << '{';
    for (auto itr=s.begin();itr!=s.end();++itr){
        os << *itr;
        if (++itr!=s.end()) os << ',';
        --itr;
    }
    os << '}';
    return os;
}

void debug_out(){cerr << '\n';}
template<class Head,class... Tail>
void debug_out(Head&& head,Tail&&... tail){
    cerr << head;
    if (sizeof...(Tail)>0) cerr << ", ";
    debug_out(move(tail)...);
}
#ifdef LOCAL
#define debug(...) cerr << " ";\
cerr << #__VA_ARGS__ << " :[" << __LINE__ << ":" << __FUNCTION__ << "]" << '\n';\
cerr << " ";\
debug_out(__VA_ARGS__)
#else
#define debug(...) 42
#endif

template<typename T> T gcd(T x,T y){return y!=0?gcd(y,x%y):x;}
template<typename T> T lcm(T x,T y){return x/gcd(x,y)*y;}

template<class T1,class T2> inline bool chmin(T1 &a,T2 b){
    if (a>b){a=b; return true;} return false;
}
template<class T1,class T2> inline bool chmax(T1 &a,T2 b){
    if (a<b){a=b; return true;} return false;
}
#pragma endregion

ll dp[42][2][2];

int main(){
    cin.tie(0);
    ios::sync_with_stdio(false);
    int N; cin >> N;
    vector<ll> A(N); cin >> A;
    ll G=0;
    for (int i=2;i<N;++i) G^=A[i];
    ll sum=A[0]+A[1];
    if (sum<G){cout << -1 << '\n'; return 0;}
    for (int i=0;i<42;++i){
        for (int j=0;j<2;++j){
            for (int k=0;k<2;++k){
                dp[i][j][k]=IINF;
            }
        }
    }
    dp[41][0][0]=0;
    map<pair<int,int>,pair<int,int>> to;
    to[{0,0}]={0,0};
    to[{0,1}]={0,1};
    to[{1,1}]={0,0};
    to[{1,0}]={1,1};
    to[{2,0}]={1,0};
    to[{2,1}]={1,1};
    int g,s,a,wa;
    for (int i=40;i>=0;--i){
        g=(G&1LL<<i)>>i,s=(sum&1LL<<i)>>i,a=(A[0]&1LL<<i)>>i;
        for (int j=0;j<2;++j){
            wa=j+(g^j);
            ll diff=a-j;
            if (!to.count({wa,s})) continue;
            auto p=to[{wa,s}];
            int k=p.first,nk=p.second;
            if (a==j) chmin(dp[i][0][nk],dp[i+1][0][k]*2);
            if (a==1&&j==0) chmin(dp[i][1][nk],dp[i+1][0][k]*2+diff);
            if (dp[i+1][1][k]*2+diff<0) continue;
            chmin(dp[i][1][nk],dp[i+1][1][k]*2+diff);
        }
    }
    ll ans=min(dp[0][0][0],dp[0][1][0]);
    cout << (ans==A[0]||ans==IINF?-1:ans) << '\n';
}

Measured memory_bytes: 3652