#![allow(unused_imports)]
#![allow(non_snake_case)]
use std::cmp::*;
use std::collections::*;
use std::io::Write;

#[allow(unused_macros)]
macro_rules! debug {
    ($($e:expr),*) => {
        #[cfg(debug_assertions)]
        $({
            let (e, mut err) = (stringify!($e), std::io::stderr());
            writeln!(err, "{} = {:?}", e, $e).unwrap()
        })*
    };
}

fn main() {
    let n = read::<usize>();
    let mut v = read_vec::<usize>();
    v.dedup();
    let mut exist = vec![false; 100001];

    let mut count = 0;
    for i in 0..v.len() {
        if exist[v[i]] {
            count += 1;
            if i != v.len() - 1 || v[i] != v[0] {
                count += 1;
            }
        }
        exist[v[i]] = true;
    }

    if count == 0 {
        println!("0");
    } else if count == 1 {
        println!("1");
    } else {
        println!("-1");
    }
}

fn read<T: std::str::FromStr>() -> T {
    let mut s = String::new();
    std::io::stdin().read_line(&mut s).ok();
    s.trim().parse().ok().unwrap()
}

fn read_vec<T: std::str::FromStr>() -> Vec<T> {
    read::<String>()
        .split_whitespace()
        .map(|e| e.parse().ok().unwrap())
        .collect()
}