fn main() {
    let v = read_vec::<usize>();
    let (n, m, a, b) = (v[0], v[1], v[2] - 1, v[3] - 1);
    let mut edges = vec![vec![]; n + 1];

    for i in 0..m {
        let v = read_vec::<usize>();
        let (l, r) = (v[0] - 1, v[1] - 1);

        let l = std::cmp::max(l, a);
        let r = std::cmp::min(b, r);
        edges[l].push((r + 1, 1));
        edges[r + 1].push((l, 1));
    }
    let d = solve(&edges, a);
    let ans = d[b + 1];
    if ans == INF {
        println!("-1");
        return;
    }
    println!("{}", ans);
}

type Cost = i64;
const INF: Cost = 100000_00000_00000;

fn solve(edges: &Vec<Vec<(usize, Cost)>>, start_idx: usize) -> Vec<Cost> {
    let num_apexes = edges.len();
    let mut d = vec![INF; num_apexes];
    d[start_idx] = 0;
    let mut que = std::collections::BinaryHeap::new();
    que.push((std::cmp::Reverse(0), start_idx));

    while let Some((u, v)) = que.pop() {
        if d[v] < u.0 {
            continue;
        }
        for e in &edges[v] {
            if d[v] != INF && d[e.0] > d[v] + e.1 {
                d[e.0] = d[v] + e.1;
                que.push((std::cmp::Reverse(d[e.0]), e.0));
            }
        }
    }
    d
}

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()
}