#define ll long long
#define dd long double
#define pub push_back
#define pob pop_back
#define puf push_front
#define pof pop_front
#define mp make_pair
#define mt make_tuple
#define fo(i , n) for(ll i = 0 ; i < n ; i++)
//#include<bits/stdc++.h>
#include<iomanip>
#include<cmath>
#include<cstdio>
#include<utility>
#include<iostream>
#include<vector>
#include<string>
#include<algorithm>
#include<map>
#include<queue>
#include<set>
#include<stack>
#include<bitset>
#include<sstream>
dd pi = acos(-1) ;
ll z =  1000000007 ;
ll inf = 1000000000000000000 ;
ll p1 = 37 ;
ll p2 = 53 ;
ll mod1 =  202976689 ;
ll mod2 =  203034253 ;
ll fact[105] ;
ll gdp(ll a , ll b){return (a - (a%b)) ;}
ll ld(ll a , ll b){if(a < 0) return -1*gdp(abs(a) , b) ; if(a%b == 0) return a ; return (a + (b - a%b)) ;} // least number >=a divisible by b
ll gd(ll a , ll b){if(a < 0) return(-1 * ld(abs(a) , b)) ;    return (a - (a%b)) ;} // greatest number <= a divisible by b
ll gcd(ll a , ll b){ if(b > a) return gcd(b , a) ; if(b == 0) return a ; return gcd(b , a%b) ;}
ll e_gcd(ll a , ll b , ll &x , ll &y){ if(b > a) return e_gcd(b , a , y , x) ; if(b == 0){x = 1 ; y = 0 ; return a ;}
ll x1 , y1 ; e_gcd(b , a%b , x1 , y1) ; x = y1 ; y = (x1 - ((a/b) * y1)) ; return e_gcd(b , a%b , x1 , y1) ;}
ll power(ll a ,ll b , ll p){if(b == 0) return 1 ; ll c = power(a , b/2 , p) ; if(b%2 == 0) return ((c*c)%p) ; else return ((((c*c)%p)*a)%p) ;}
ll inverse(ll a ,ll n){return power(a , n-2 , n) ;}
ll max(ll a , ll b){if(a > b) return a ; return b ;}
ll min(ll a , ll b){if(a < b) return a ; return b ;}
ll left(ll i){return ((2*i)+1) ;}
ll right(ll i){return ((2*i) + 2) ;}
//ll ncr(ll n , ll r){if(n < r|| (n < 0) || (r < 0)) return 0 ; return ((((fact[n] * inv_fact[r])%z) * inv_fact[n-r]) % z);}
void swap(ll&a , ll&b){ll c = a ; a = b ; b = c ; return ;}
//ios_base::sync_with_stdio(0);
//cin.tie(0); cout.tie(0);
using namespace std ;
#include <ext/pb_ds/assoc_container.hpp> 
#include <ext/pb_ds/tree_policy.hpp> 
using namespace __gnu_pbds; 
#define ordered_set tree<int, null_type,less<int>, rb_tree_tag,tree_order_statistics_node_update>
// ordered_set s ; s.order_of_key(val)  no. of elements strictly less than val
// s.find_by_order(i)  itertor to ith element (0 indexed)
//__builtin_popcount(n) -> returns number of set bits in n
// ll seed;
//mt19937 rnd(seed=chrono::steady_clock::now().time_since_epoch().count()); // include bits
#define tll tuple<ll , ll ,ll , ll>
#define pll pair<ll ,ll>


void solve()
{
    ll n ;
    cin >> n ;
    ll arr[n] ;
    fo(i , n)
    {
        cin >> arr[i] ;
    }
    ll ans = 0 ;
    for(ll i = 2 ; i < n ; i++)
    {
        ans = (ll)(ans ^ arr[i]) ;
    }
    ll a = arr[0] , b = arr[1] , c = ans ;
    ll d = (a+b)-c ;
    if(d%2 == 1)
    {
        cout << -1 << endl ;
        return ;
    }
    d /= 2 ;

    ll ava = (a+b - 2*d) ;
    a = d ;
    b = d ;

    ll curr = power(2 , 50, inf) ;
    while(curr > 0)
    {
        if(((ll)(curr & ava)) != 0)
        {
            if(a + curr <= arr[0])
            {
                a += curr ;
            }
            else
            {
                b += curr ;
            }
            ava -= curr ;
        }
        curr /= 2 ;
    }
    if(a == 0)
    {
        cout << -1 << endl ;
        return ;
    }
    ll fin = ((ll)(a^b)) ;
    if(fin != ans)
    {
        cout << -1 << endl ;
        return ;
    }
    cout << max(arr[0] - a , -1) << endl ;
    return ;
}

int main()
{
    /*ios_base::sync_with_stdio(0);
    cin.tie(0); cout.tie(0);
    #ifndef ONLINE_JUDGE
    freopen("inputf.txt" , "r" , stdin) ;
    freopen("outputf.txt" , "w" , stdout) ;
    freopen("error.txt" , "w" , stderr) ;
    #endif*/
    
    ll t ;
    //cin >> t ;
    t = 1 ;
    
    

    while(t--)
    {
        solve() ;
        
    }
    //cerr << "Time : " << 1000 * ((double)clock()) / (double)CLOCKS_PER_SEC << "ms\n";
 
    return 0;
}

Measured memory_bytes: 3628