#include <stdio.h>
#include <atcoder/all>
#include <bits/stdc++.h>
using namespace std;
using namespace atcoder;
using mint = modint998244353;
#define rep(i,n) for (int i = 0; i < (n); ++i)
#define Inf32 1000000005
#define Inf64 1000000000000000001LL

int main(){
	
	int n;
	cin>>n;
	vector<int> b(n);
	rep(i,n)cin>>b[i];
	
	vector<int> f(20);
	rep(i,n){
		int ii = i+1;
		rep(j,20){
			if((ii>>j)&1){
				f[j] |= 1<<((b[i]>>j)&1);
			}
			else{
				if((b[i]>>j)&1){
					cout<<-1<<endl;
					return 0;
				}
			}
		}
	}
	rep(i,20){
		if(f[i]==3){
			cout<<-1<<endl;
			return 0;
		}
	}
	int ans = 0;
	rep(i,20){
		if(f[i]==2)ans |= 1<<i;
	}
	if(ans==0){
		rep(i,20){
			if(f[i]==0){
				ans |= 1<<i;
				break;
			}
		}
	}
	cout<<ans<<endl;
}