use std::collections::VecDeque;

fn main() {
    let (n, m) = {
        let mut line = String::new();
        std::io::stdin().read_line(&mut line).unwrap();
        let mut iter = line.split_whitespace();
        (
            iter.next().unwrap().parse::<usize>().unwrap(),
            iter.next().unwrap().parse::<usize>().unwrap(),
        )
    };
    let uv: Vec<_> = (0..m)
        .map(|_| {
            let mut line = String::new();
            std::io::stdin().read_line(&mut line).unwrap();
            let mut iter = line.split_whitespace();
            (
                iter.next().unwrap().parse::<usize>().unwrap() - 1,
                iter.next().unwrap().parse::<usize>().unwrap() - 1,
            )
        })
        .collect();

    let mut graph = vec![vec![]; n];
    for &(u, v) in &uv {
        graph[u].push(v);
    }

    let u = n - 2;
    let v = n - 1;

    let dists_0 = calc_distances(&graph, 0);
    let dists_u = calc_distances(&graph, u);
    let dists_v = calc_distances(&graph, v);

    let mut ans = None;

    if let (Some(d1), Some(d2), Some(d3)) = (dists_0[u], dists_u[v], dists_v[0]) {
        update_cost(&mut ans, d1 + d2 + d3);
    }

    if let (Some(d1), Some(d2), Some(d3)) = (dists_0[v], dists_v[u], dists_u[0]) {
        update_cost(&mut ans, d1 + d2 + d3);
    }

    if let (Some(d1), Some(d2), Some(d3), Some(d4)) =
        (dists_0[u], dists_u[v], dists_v[u], dists_u[0])
    {
        update_cost(&mut ans, d1 + d2 + d3 + d4);
    }

    if let (Some(d1), Some(d2), Some(d3), Some(d4)) =
        (dists_0[v], dists_v[u], dists_u[v], dists_v[0])
    {
        update_cost(&mut ans, d1 + d2 + d3 + d4);
    }

    match ans {
        Some(ans) => println!("{}", ans),
        None => println!("-1"),
    }
}

fn update_cost(cost: &mut Option<usize>, cand_cost: usize) {
    if cost.is_none() || cand_cost < cost.unwrap() {
        *cost = Some(cand_cost);
    }
}

fn calc_distances(graph: &[Vec<usize>], start: usize) -> Vec<Option<usize>> {
    let n = graph.len();

    let mut dists = vec![None; n];

    let mut queue = VecDeque::from([(start, 0)]);
    while let Some((cur, dist)) = queue.pop_front() {
        if dists[cur].is_some() {
            continue;
        }

        dists[cur] = Some(dist);

        for &next in &graph[cur] {
            queue.push_back((next, dist + 1));
        }
    }

    dists
}