use proconio::input;

fn main() {
    input! {
        n: usize,
        b: [usize; n],
    }

    let mut ans = 0;
    for k in 0..20 {
        let Some(bit) = (0..2).find(|&bit| {
            b.iter()
                .enumerate()
                .all(|(i, &b)| (i + 1) >> k & bit == b >> k & 1)
        }) else {
            println!("-1");
            return;
        };
        ans += bit << k;
    }

    if ans == 0 {
        ans = (n + 1).next_power_of_two();
    }

    println!("{ans}");
}