#include <ext/pb_ds/assoc_container.hpp> 
#include <ext/pb_ds/tree_policy.hpp>  
#include <functional>
#include <iostream> 
#include<bits/stdc++.h>
using namespace __gnu_pbds;  
using namespace std;  
// Declaring ordered_set 
typedef tree<long long int, null_type, less<long long int>, rb_tree_tag,  
            tree_order_statistics_node_update>  
    ordered_set;  
// https://www.geeksforgeeks.org/order_of_key-in-c/
 
typedef pair<long long int,long long int> pa;
 
#define resize(A) A.resize(unique(A.begin(), A.end()) - A.begin())
#define pb push_back
#define  MAX 200005

#define endl "\n"                   //Take care in case of interactive problems


long long int mod=998244353;
long long int MOD=1e9+7;
long long int INF=1e18;
 
 
 
 
////////////////////////////////////////////// sieve  //////////////////////////////////////////////////////////////////

/*bool C[MAX];
vector<long long  int>V;
 void sieve()
 {
    long long int i,j;
    for(i=2;i<MAX;i++)
    {
      if(!C[i])
     { 
         
          V.pb(i);
        for(j=2*i;j<MAX;j=j+i)
        {
            C[j]=true;
        }
      }
        
    }
     
 }
 
 */
 
 
//////////////////////////////////////////////////////////D-S-U/////////////////////////////////////////////////////////
/*long long int parent[MAX];
long long int size[MAX];
void initialise()
{ //cout<<MAX<<" ";
    for(long  long int i=0;i<MAX;i++)
    {
        parent[i]=i;
        //cout<<i<<" ";
        size[i]=1;
    }
}
long long int  root(long long int x)
{
    while(x!=parent[x])
    {
        parent[x]=parent[parent[x]];
        x=parent[x];
    }
    return x;
}
 
    
void union1(long long int x,long long int y)
{
    if(size[x]>=size[y])
    {
        size[x]+=size[y];
        parent[y]=x;
        
    }
    else
    {
        size[y]+=size[x];
        parent[x]=y;
    }
}*/
//////////////////////////////////////////////////////MODPOWER//////////////////////////////////////////////////////////
 
 
long long int modulo(long long int a,long long  int b, long long int n){
    long long int  x=1, y=a%n; 
    while (b > 0) {
        if (b%2 == 1) {
            x = (x*y) % n; // multiplying with base
        }
        y = (y*y) % n; // squaring the base
        b /= 2;
    }
    return x % n;
}
 
 
////////////////////////////////////////////////BIT/////////////////////////////////////////////////////////////////////
/* long long int bit[MAX+1]={0};
long long int query(long long int in,long long int n)
{
    //in=in+1;
    long long int sum=0;
    if(in==0)
        return 0;
    while(in>0)
    {
      sum=sum+bit[in];
        
        in=in-(in&(-in));
    }
    return sum;
}
void update(long long int in,long long int n,long long int val)
{
    // in=in+1;
    
    while(in<=n)
    {
       // cout<<in<<" ";
        bit[in]+=val;
        in=in+ (in &(-in));
    }
}
 
 */
 

//////////////////////////////////////////////// DFS //////////////////////////////////////////////////////////////////
/*
  bool visited[MAX];
 long long int parent[MAX];
 long long int dist[MAX];


void dfs(vector<long long int>adj[],long long int i,long long int k)
{
    visited[i]=true;
    for(auto j:adj[i])
    {
        if(!visited[j])
        {
            dist[j]=dist[i]+1;
            parent[j]=i;
            dfs(adj,j,k);
            
        }
        
    }
}
 */
 

////////////////////////////////////////////////////////////////////////////////////////////////////////////////////////

 
long long int R[4]={1,0,-1,0};
long long int C[4]={0,1,0,-1};
 
 


/*
           Come Rain, Come Snow.
           Wherever your dreams take you, you gotta go.
*/
long long  int ax = 0, bx = 0; 
long long int  P=0;
void compute(long long  int S, long long int X,long long int z) 
{ 
    if((S-X)%2==1)
      P=1;
    
     long long int A = (S - X)/2;
  
      long long int a=0;
      long long int b=0,i;
    long long int B[45];B[0]=1;
    for(i=1;i<45;i++)
    {
        B[i]=B[i-1]*2;
    }
      vector<long long int>V;
   
    for(i=44;i>=0;i--)
    {
         long long int k=((X & (B[i])));
         long long int l=((A & (B[i])));
         if (k == 0 && l == 0) 
        { 
            // Let us leave bits as 0. 
        } 
        else if (k == 0 && l > 0) 
        { 
            a +=B[i];  
            b +=B[i];  
        } 
        else if (k > 0 && l == 0) 
        { 
          
            V.pb(i);
  
           
        } 
        else // (Xi == 1 && Ai == 1) 
        { 
              P=1; 
            
        } 
    }
    if(a>z)
        P=1;
    else
    {
        for(i=0;i<V.size();i++)
        {
            if(a+B[V[i]]<=z)
                a=a+B[V[i]];
            else
                b=b+B[V[i]];
        }
        
    }
    ax=a;
    bx=b;
    
} 
  



int main()
{
 ios_base::sync_with_stdio(false); 
 cin.tie(NULL);
 cout.tie(NULL);
long long int  sum,d,i,w,j,k=0,l=0,r=0,m,n,p=0,cnt=0,cnt1=0,x,y=0,z=0,a,b,c,low,mid,high,ans=0,ans1=0,maxi=0,mini=INF,t1,x1,y1,z1,t,o,i1,j1;
string s,s1,s2,s3,s4;
    
    cin>>n;
    long long int A[n];
    for(i=0;i<n;i++)
    {
        cin>>A[i];
    }
    x=0;
for(i=2;i<n;i++)
     x=x^A[i];

    a=x;
    b=A[0];
    c=A[1];
     compute(b+c,x,b);
  if(ax==0)
    P=1;
    if(P)
        cout<<-1;
    else
        cout<<bx-c<<endl;
 
}


Measured memory_bytes: 3576