#include<bits/stdc++.h>
#include<stdio.h>
using namespace std;
typedef long long int ll;
typedef long double ld;
typedef unsigned long long int lu;
typedef vector< int > vi;
typedef vector < ll > vl;
typedef set <ll> sl;
typedef vector< vi > vvi;
typedef vector< vl > vvl;
typedef pair< int,int > ii;
typedef pair< ll,ll > il;
#define sz(a) ll((a).size())
#define test(t) ll t;cin>>t;while(t--)
#define pb push_back
#define all(c) (c).begin(),(c).end()
#define tr(c,i) for(__typeof((c).begin()) i = (c).begin(); i != (c).end(); i++)
#define present(c,x) ((c).find(x) != (c).end())
#define cpresent(c,x) (find(all(c),x) != (c).end())
#define fastio ios_base::sync_with_stdio(0);cin.tie(0);cout.tie(0)
#define Vi vector
#define pr pair
#define mpr make_pair
#define fi first
#define se second
#define isnum(a) (a>=48 && a<=57)
#define isS(a) (a>='a' && a<='z')
#define isU(a) (a>='A' && a<='Z')
#define toS(a) (isU(a)?a+32:a)
#define toU(a) (isS(a)?a-32:a)
#define toC(a) a&15
 
vector <ll> sprime;
ll d[100005];
 
vector<char> prime (1000000+5, true);
void sieve(ll n)
{   
    //O(nloglogn) time
    prime[0] = prime[1] = false;
    for (ll i=2; i<=n; ++i)
    {
        if (prime[i])
        {
            sprime.pb(i);           
            for (ll j=2*i; j<=n; j+=i)
            {
                prime[j] = false;
            }
        }
    }
}
#define forn(i,e) for(int i = 0; i < e; i++)
#define rep(i,a,b) for(i=a;i<b;i++)
#define REP(i,a,b) for(i=a;i<=b;i++)
#define reprev(i,a,b) for(i=a;i>=b;i--)
 
// Convex Hull Trick
typedef ll LT;
typedef pair<LT,LT> LP;
struct CHT {
  deque<LP> d;
  LT get(LT x) {
    while (sz(d) >= 2) {
      LT a = d[0].fi*x + d[0].se;
      LT b = d[1].fi*x + d[1].se;
      if (a <= b) break; // get min
      d.pop_front();
    }
    return d[0].fi*x + d[0].se;
  }
  void add(LP x) { // decreasing order of x.fi
    while (sz(d) >= 2) {
      LP y = d[sz(d) - 1];
      LP z = d[sz(d) - 2];
      if ((x.se-y.se)*(z.fi-y.fi) >= (y.fi-x.fi)*(y.se-z.se)) break; // both
      d.pop_back();
    }
    d.pb(x);
  }
};
//
 
#define leadzero(a) __builtin_clz(a) //count leading zeros
#define trailzero(a) __builtin_ctz(a) //count trailing zeros
#define bitcount(a) __builtin_popcount(a) // count set bits (add ll)
#define ln cout<<endl
#define sp cout<<" "
#define maxe(a) *max_element(a.begin(),a.end())
#define maxi(a) max_element(a.begin(),a.end())-a.begin()
#define mine(a) *min_element(a.begin(),a.end())
#define mini(a) min_element(a.begin(),a.end())-a.begin()
ll gcd(ll a, ll b) { if(b == 0) return a; return gcd(b, a % b); }
ll fastpowMOD(ll a, ll p,ll MOD){ if(p==0) return 1; ll z = fastpowMOD(a,p/2,MOD); z = (z*z)%MOD; if(p%2) z = (z*a)%MOD; return z; }
ll fastpow(ll a, ll p){ if(p==0) return 1; ll z = fastpow(a,p/2); z = (z*z); if(p%2) z = (z*a); return z; }
//CHT stands for convex hull trick
ll search1(ll a[], ll f,ll n)
{
    ll left=1;
    ll right=n+1;
    while(right-left>1)
    {
        ll mid = left + (right-left)/2;
        if(a[mid]<f)
        {
            left=mid;
        }
        else right=mid;
    }
    return a[left];
}
 
const ll INF = 1e18;
vector<ll> lazy;
vector<ll> tree;
 
void updateRange(ll node, ll start, ll end, ll l, ll r, ll val)
{
    if(lazy[node] != 0)
    { 
        // This node needs to be updated
        tree[node] += (/*end - start*/ + 1) * lazy[node];    // Update it
        if(start != end)
        {
            lazy[node*2] += lazy[node];                  // Mark child as lazy
            lazy[node*2+1] += lazy[node];                // Mark child as lazy
        }
        lazy[node] = 0;                                  // Reset it
    }
    if(start > end or start > r or end < l)              // Current segment is not within range [l, r]
        return;
    if(start >= l and end <= r)
    {
        // Segment is fully within range
        lazy[node] += (/*end - start*/ + 1) * val;
        if(start != end)
        {
            // Not leaf node
            lazy[node*2] += lazy[node];
            lazy[node*2+1] += lazy[node];
        }
        tree[node]+=lazy[node];
        lazy[node]=0;
        return;
    }
    ll mid = (start + end) / 2;
    updateRange(node*2, start, mid, l, r, val);        // Updating left child
    updateRange(node*2 + 1, mid + 1, end, l, r, val);   // Updating right child
    tree[node] = max(tree[node*2],tree[node*2+1]);        // Updating root with max value 
}
 
ll queryRange(ll node, ll start, ll end, ll l, ll r)
{
    if(lazy[node] != 0)
    {
        // This node needs to be updated
        tree[node] += (/*end - start +*/ 1) * lazy[node];            // Update it
        if(start != end)
        {
            lazy[node*2] += lazy[node];         // Mark child as lazy
            lazy[node*2+1] += lazy[node];    // Mark child as lazy
        }
        lazy[node] = 0;                 // Reset it
    }
 
    if(start > end or start > r or end < l)
        return -INF;         // Out of range
 
    if(start >= l and end <= r)             // Current segment is totally within range [l, r]
        return tree[node];
    ll mid = (start + end) / 2;
    if(r<=mid)return queryRange(node*2, start, mid, l, r);
    else if(l>=mid+1) return queryRange(node*2 + 1, mid + 1, end, l, r);
    ll p1 = queryRange(node*2, start, mid, l, r);         // Query left child
    ll p2 = queryRange(node*2 + 1, mid + 1, end, l, r); // Query right child
    //tree[node] = max(tree[node*2],tree[node*2+1]);
    return max(p1,p2);
}
void convert_reduce(ll arr[], ll n) 
{ 
    vector <pair<ll, ll> > v; 
    for (ll i = 0; i < n; i++)v.push_back(make_pair(arr[i], i)); 
    sort(v.begin(), v.end());
    ll cnt = 0; 
    for (ll i=0; i<n; i++)
    {
        arr[v[i].second] = cnt;
        ll j = i;
        while(j<n && v[j].fi==v[i].fi)arr[v[j++].second] = cnt;
        i=j-1;
        cnt++;
    } 
} 

//for bfs 
ll dir[4][2] = {{1, 0}, {-1, 0}, {0, 1}, {0, -1}};

bool valid(ll i, ll j,ll n,ll m) { return i >= 1 && i <= n && j >= 1 && j <= m; }

ll MOD = 998244353;

int main(){
    fastio;
    ll n,k,i,j;
    cin>>n;
    ll a[n];
    rep(i,0,n)cin>>a[i];
    ll X,S;
    X=0;
    rep(i,2,n)X^=a[i];
    S = a[0]+a[1];
    if(S<X || (S-X)%2)
    {
        cout<<-1<<endl;
        return 0;
    }
    ll A = (S-X)/2;
    ll bits0[42];
    memset(bits0,0,sizeof(bits0));
    ll temp = a[0];
    ll  maxbit=-1;
    rep(i,0,42)
    {
        if(temp&1)
        {
            bits0[i]=1;
            maxbit = i;
        }
        else bits0[i]=0;
        temp/=2;
    }
    ll bitsa[42],bitsb[42];
    memset(bitsa,-1,sizeof(bitsa));
    memset(bitsb,-1,sizeof(bitsb));
    vl v;
    bool lt=true;
    ll lat0=-1;
    ll lat1=-1;
    rep(i,0,42)
    {
        if(!(X>>i&1) && (A>>i&1))
        {
            bitsa[i]=bitsb[i]=1;
            if(bits0[i]==0 && bitsa[i]==1)lat0=i;
            if(i>maxbit){lt=false;break;}
        }
        else if(!(X>>i&1) && !(A>>i&1))
        {
            bitsa[i]=bitsb[i]=0;
            if(bits0[i]==1 && bitsa[i]==0)lat1=i;   
        }
        else if((X>>i&1) && !(A>>i&1))
        {
            if(i<=maxbit)v.pb(i);
            else
            {
                bitsa[i]=0;
                bitsb[i]=1;
            }
        }
        else
        {
            lt=false;
            break;
        }
    }
    if(!lt)
    {
        cout<<-1<<endl;
        return 0;
    }
    if(lat1>lat0)
    {
        rep(i,0,sz(v))
        {
            if(v[i]<lat1)bitsa[v[i]]=1;
            else bitsa[v[i]]=bits0[v[i]];
        }
    }
    else if(lat0>lat1)
    {
        bool lt1=false;
        rep(i,0,sz(v))
        {
            if(lt1)bitsa[v[i]]=bits0[v[i]];
            else
            {
                if(v[i]<lat0)bitsa[v[i]]=1;
                else 
                {
                    if(bits0[v[i]]==1)
                    {
                        bitsa[v[i]]=0;
                        lt1=true;
                    }
                    else
                    {
                        bitsa[v[i]]=0;
                    }
                }
            }
        }
        if(!lt1)
        {
            cout<<-1<<endl;
            return 0;
        }
    }
    else
    {
        rep(i,0,sz(v))
        {
            bitsa[v[i]]=bits0[v[i]];
        }
    }
    ll val = 0;
    ll prod=1;
    rep(i,0,42)
    {
        if(bitsa[i]==-1)break;
        if(bitsa[i]==1)val+=prod;
        prod*=2;
    }
    if(i!=42)
    {
        cout<<-1<<endl;
        return 0;
    }
    ll ans = a[0]-val;
    if(ans<a[0] && ans>=0)
    {
        cout<<ans<<endl;
    }
    else cout<<-1<<endl;
    return 0;
}

Measured memory_bytes: 4124