use std::vec;

macro_rules! input {
    ($($r:tt)*) => {
        let stdin = std::io::stdin();
        let mut bytes = std::io::Read::bytes(std::io::BufReader::new(stdin.lock()));
        let mut next = move || -> String{
            bytes.by_ref().map(|r|r.unwrap() as char)
                .skip_while(|c|c.is_whitespace())
                .take_while(|c|!c.is_whitespace())
                .collect()
        };
        input_inner!{next, $($r)*}
    };
}

macro_rules! input_inner {
    ($next:expr) => {};
    ($next:expr,) => {};
    ($next:expr, $var:ident : $t:tt $($r:tt)*) => {
        let $var = read_value!($next, $t);
        input_inner!{$next $($r)*}
    };
}

macro_rules! read_value {
    ($next:expr, ( $($t:tt),* )) => { ($(read_value!($next, $t)),*) };
    ($next:expr, [ $t:tt ; $len:expr ]) => {
        (0..$len).map(|_| read_value!($next, $t)).collect::<Vec<_>>()
    };
    ($next:expr, $t:ty) => ($next().parse::<$t>().expect("Parse error"));
}

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

    let graph = {
        let mut graph = vec![vec![]; n];
        for &(a, b) in &edges {
            let a = a - 1;
            let b = b - 1;
            graph[a].push(b);
        }
        graph
    };

    let inf = 998244353;
    let get_dist = |start: usize| -> Vec<usize> {
        let mut dist = vec![inf; n];
        dist[start] = 0;
        let mut queue = std::collections::VecDeque::new();
        queue.push_back(start);
        while let Some(v) = queue.pop_front() {
            let d = dist[v];
            for &e in &graph[v] {
                if dist[e] <= d + 1 {
                    continue;
                }
                dist[e] = d + 1;
                queue.push_back(e);
            }
        }
        dist
    };

    let dist_st = get_dist(0);
    let dist_a = get_dist(n - 2);
    let dist_b = get_dist(n - 1);

    let st_to_a = dist_st[n - 2];
    let st_to_b = dist_st[n - 1];
    let a_to_b = dist_a[n - 1];
    let a_to_st = dist_a[0];
    let b_to_a = dist_b[n - 2];
    let b_to_st = dist_b[0];

    let ans = {
        let ans = (st_to_a + a_to_b + b_to_st).min(st_to_b + b_to_a + a_to_st);
        let mut ans = ans as i64;
        if ans >= inf as i64 {
            ans = -1;
        }
        ans
    };

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