use proconio::input;

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

    let mut seen = vec![false; n + 1];
    let mut prev = 0;
    let mut ok = true;
    for i in 0..n {
        if prev != a[i] && seen[a[i]] {
            ok = false;
            break;
        }

        seen[a[i]] = true;
        prev = a[i];
    }
    if ok {
        println!("0");
        return;
    }

    let mut poss = vec![vec![]; n + 1];
    for i in 0..n {
        poss[a[i]].push(i as i64);
    }

    let mut ok = true;
    for i in 0..=n {
        if poss[i].len() <= 1 {
            continue;
        }

        if !f(n, poss[i].len(), &poss[i]) {
            ok = false;
        }
    }

    let ans = if ok { 1 } else { -1 };

    println!("{}", ans);
}

fn f(m: usize, n: usize, a: &Vec<i64>) -> bool {
    for i in 0..n {
        let l = (a[i] - a[(i as i64 - 1).rem_euclid(n as i64) as usize]).rem_euclid(m as i64);
        let r = (a[(i + 1) % n] - a[i]).rem_euclid(m as i64);
        if l != 1 && r != 1 {
            return false;
        }
    }

    true
}