fn main() {
    let mut n = String::new();
    std::io::stdin().read_line(&mut n).ok();
    let n: usize = n.trim().parse().unwrap();
    let mut a = String::new();
    std::io::stdin().read_line(&mut a).ok();
    let a: Vec<usize> = a.trim().split_whitespace().map(|s| s.parse().unwrap()).collect();

    let mut colors: Vec<Vec<usize>> = vec![vec![]; n+1];
    let mut prev_color: usize = n+1;
    a.iter().enumerate().for_each(|pair| {
        if prev_color == *pair.1 {
            return;
        }
        colors[*pair.1].push(pair.0);
        prev_color = *pair.1;
    });
    let mut result: usize = 0;
    for i in 1..=n {
        let v = &colors[i];
        if v.len() > 2 {
            println!("-1");
            return;
        } else if v.len() == 2 {
            if v[0] == 0 && i == prev_color {
                result += 1;
            } else {
                println!("-1");
                return;
            }
        }
    }
    println!("{}", result);
}