#include <bits/stdc++.h>
#define be(v) (v).begin(),(v).end()
#define pb(q) push_back(q)
typedef long long ll;
using namespace std;
const ll mod=1000000007, INF=(1LL<<45);
#define doublecout(a) cout<<fixed<<setprecision(10)<<a<<endl;


int main() {
    cin.tie(0);
    cout.tie(0);
    ios::sync_with_stdio(false);
    ll n, x, a, b, sum = 0;
    cin >> n;
    for(int i=0;i<n;i++){
        if(i == 0) cin >> a; 
        else if(i == 1) cin >> b;
        else {
        	cin >> x;
        	sum ^= x;
        }
    }
    ll num = a + b - sum;
    if(num < 0LL || (num & 1)){
    	cout << -1 << endl;
    	return 0;
    }
    num /= 2LL;
    vector<ll> c(45, -1LL);
    for(int i=0;i<45;i++) if((sum >> i & 1) == 0){
    	if(num >> i & 1) c[i] = 1;
    	else c[i] = 0;
    }
    ll ans = 0;
    for(int i=0;i<45;i++) if(c[i] == 1LL) ans += 1LL << i;
    for(int i=44;i>=0;i--) if(c[i] == -1LL){
        if(ans + (1LL << i) <= a) ans += 1LL << i;
    }
    num = b + a - ans;

    if(ans == 0LL || ans > a || (num ^ ans) != sum) cout << -1 << endl;
    else cout << a - ans << endl;
    return 0;
}
