use std::io::Read;

fn run() {
    let mut s = String::new();
    std::io::stdin().read_to_string(&mut s).unwrap();
    let mut it = s.trim().split_whitespace();
    let n: usize = it.next().unwrap().parse().unwrap();
    let mut a: Vec<usize> = it.map(|s| s.parse().unwrap()).collect();
    a.dedup();
    let mut id = vec![n; n + 1];
    let mut ans = 0;
    for (i, &x) in a.iter().enumerate() {
        if id[x] != n {
            if a[0] == x && i == a.len() - 1 {
                ans = 1;
            } else {
                ans = -1;
                break;
            }
        } else {
            id[x] = i;
        }
    }
    println!("{}", ans);
}

fn main() {
    run();
}