// writer: w33z8kqrqk8zzzx33
#include <bits/stdc++.h>
using namespace std;

#define iter(i, a, b) for(int i=(a); i<(b); i++)
#define rep(i, a) iter(i, 0, a)
#define rep1(i, a) iter(i, 1, (a)+1)
#define fi first
#define se second
#define pb push_back
 
#define ll long long
#define pii pair<int, int>
#define int ll
const int MOD = 1000000007;

bool isAuspicious(int a, int b, int l, int r, int x) {
	x >>= 21;
	int x1 = (a-l) >> 21;
	int x2 = (a-r) >> 21;
	
	int y1 = (b+l) >> 21;
	int y2 = (b+r) >> 21;
	return ((x1^y1)==x) || ((x1^y2)==x) || ((x2^y1)==x) || ((x2^y2)==x);
}

signed main() {
    ios_base::sync_with_stdio(false); cin.tie(0);
    int n; cin >> n;
    int a, b, x = 0; cin >> a >> b;
    rep(i, n-2) {
    	int p; cin >> p;
    	x ^= p;
    }
    for(int l = 0; l < a; l += (1 << 20)) {
    	int r = min(l + (1 << 20) - 1, a - 1);
    	if(!isAuspicious(a, b, l, r, x)) continue;
    	iter(ct, l, r+1) {
    		if(((a-ct) ^ (b+ct)) == x) {
    			cout << ct << endl;
    			return 0;
    		}
    	}
    }
    cout << -1 << endl;
}

