import java.util.Scanner;
public class Hanoi_Tower  {
    public static void main(String[] args) {
        // TODO Auto-generated method stub
        Scanner sc = new Scanner(System.in);
        int n = sc.nextInt();
        int d[] = new int [n+1];
        for (int i = 1; i <= n; i++) {
            d[i] = sc.nextInt();
        }
        int x =n%2;
        int a[][] ={{3, 1, 2, 3}, {2, 1, 3, 2}};
        double ans = 0;
        for (int i = n; i > 0; i--) {
            int y = (i+x)%2;
            double rep = (ans/Math.pow(2, i) + 1)%3;
                if (d[i] != a[y][(int)rep]){
                    if (d[i] != a[y][(int)(rep+1)]){
                        System.out.println("-1");
                        return;
                    }
                    ans += Math.pow(2, (i-1));
                }
        }
        System.out.printf("%.0f",ans);
    }
}