use std::cmp;
use std::collections::VecDeque;

fn main() {
    let (r, w) = (std::io::stdin(), std::io::stdout());
    let mut sc = IO::new(r.lock(), w.lock());

    let n: usize = sc.read();
    let first = sc.read::<usize>() - 1;
    let second = sc.read::<usize>() - 1;
    let mut graph = vec![vec![]; n];
    for _ in 1..n {
        let a: usize = sc.read::<usize>() - 1;
        let b: usize = sc.read::<usize>() - 1;
        graph[a].push(b);
        graph[b].push(a);
    }

    let mut second_dist = vec![n; n];
    second_dist[second] = 0;
    let mut q = VecDeque::new();
    q.push_back(second);
    while let Some(v) = q.pop_front() {
        for &next in graph[v].iter() {
            if second_dist[next] > second_dist[v] + 1 {
                second_dist[next] = second_dist[v] + 1;
                q.push_back(next);
            }
        }
    }

    let mut first_dist = vec![None; n];
    first_dist[first] = Some(0);
    let mut q = VecDeque::new();
    q.push_back(first);
    while let Some(v) = q.pop_front() {
        for &next in graph[v].iter() {
            if first_dist[next].is_none() {
                let d = first_dist[v].unwrap() + 1;
                if d <= second_dist[next] {
                    first_dist[next] = Some(d);
                }
                if d < second_dist[next] {
                    q.push_back(next);
                }
            }
        }
    }

    let mut ans = 0;
    for i in 0..n {
        if let Some(d) = first_dist[i] {
            ans = cmp::max(ans, d);
        }
    }

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

pub struct IO<R, W: std::io::Write>(R, std::io::BufWriter<W>);

impl<R: std::io::Read, W: std::io::Write> IO<R, W> {
    pub fn new(r: R, w: W) -> IO<R, W> {
        IO(r, std::io::BufWriter::new(w))
    }
    pub fn write<S: std::ops::Deref<Target = str>>(&mut self, s: S) {
        use std::io::Write;
        self.1.write(s.as_bytes()).unwrap();
    }
    pub fn read<T: std::str::FromStr>(&mut self) -> T {
        use std::io::Read;
        let buf = self
            .0
            .by_ref()
            .bytes()
            .map(|b| b.unwrap())
            .skip_while(|&b| b == b' ' || b == b'\n' || b == b'\r' || b == b'\t')
            .take_while(|&b| b != b' ' && b != b'\n' && b != b'\r' && b != b'\t')
            .collect::<Vec<_>>();
        unsafe { std::str::from_utf8_unchecked(&buf) }
            .parse()
            .ok()
            .expect("Parse error.")
    }
    pub fn vec<T: std::str::FromStr>(&mut self, n: usize) -> Vec<T> {
        (0..n).map(|_| self.read()).collect()
    }
    pub fn chars(&mut self) -> Vec<char> {
        self.read::<String>().chars().collect()
    }
}
