package no3249_and;
import java.util.*;
public class Main {

	public static void main(String[] args) {
		// TODO 自動生成されたメソッド・スタブ
		Scanner sc = new Scanner(System.in);
		int n = sc.nextInt();
		int[] stand = new int[30];
		boolean[] ban = new boolean[30];
		int[] two = new int[30];
		two[0] = 1;
		for(int i = 1;i < 30;i++) {
			two[i] = two[i - 1] * 2;
		}
		for(int i = 1;i <= n;i++) {
			int a = sc.nextInt();
			for(int j = 0;j < 30;j++) {
				if((i & two[j]) != 0 && (a & two[j]) == 0) {
					ban[j] = true;
				}if((i & two[j]) != 0 && (a & two[j]) != 0) {
					stand[j] = 1;
				}if((a & two[j]) != 0 && (i & two[j]) == 0) {
					System.out.print(-1);
					return;
				}
			}
		}int ans = 0;
		for(int i = 0;i < 30;i++) {
			if(stand[i] == 1 && ban[i]) {
				System.out.print(-1);
				return;
			}else if(stand[i] == 1) {
				ans += two[i];
			}
		}System.out.print(ans);
	}

}