//  ____            _                  _                       _       _
// |  _ \ _   _ ___| |_    ___ _ __   | |_ ___ _ __ ___  _ __ | | __ _| |_ ___
// | |_) | | | / __| __|  / __| '_ \  | __/ _ \ '_ ` _ \| '_ \| |/ _` | __/ _ \
// |  _ <| |_| \__ \ |_  | (__| |_) | | ||  __/ | | | | | |_) | | (_| | ||  __/
// |_| \_\\__,_|___/\__|  \___| .__/___\__\___|_| |_| |_| .__/|_|\__,_|\__\___|
//                            |_| |_____|               |_|

//https://github.com/manta1130/Competitive_Programming_Template_Rust

#[macro_use]
mod input {

    use std;
    use std::io;

    const SPLIT_DELIMITER: char = ' ';

    #[macro_export]
    #[allow(unused_macros)]
    macro_rules! input {
    ( $($x:expr ),*) => {
        {
            let temp_str = input_line_str();
            let mut split_result_iter = temp_str.split_whitespace();
                $(
                let buf_split_result = split_result_iter.next();
                let buf_split_result = buf_split_result.unwrap();
                    ($x) = buf_split_result.parse().unwrap();
                )*
        }
    };
}

    #[allow(dead_code)]
    pub fn input_line_str() -> String {
        let mut s = String::new();
        io::stdin().read_line(&mut s).unwrap();
        s.trim().to_string()
    }

    #[allow(dead_code)]
    pub fn p<T>(t: T)
    where
        T: std::fmt::Display,
    {
        println!("{}", t);
    }

    #[allow(dead_code)]
    pub fn input_vector2d<T>(line: usize) -> Vec<Vec<T>>
    where
        T: std::str::FromStr,
    {
        let mut v: Vec<Vec<T>> = Vec::new();

        for _ in 0..line {
            let vec_line = input_vector();
            v.push(vec_line);
        }
        v
    }

    #[allow(dead_code)]
    pub fn input_vector<T>() -> Vec<T>
    where
        T: std::str::FromStr,
    {
        let mut v: Vec<T> = Vec::new();

        let s = input_line_str();
        let split_result = s.split(SPLIT_DELIMITER);
        for z in split_result {
            let buf = match z.parse() {
                Ok(r) => r,
                Err(_) => panic!("Parse Error"),
            };
            v.push(buf);
        }
        v
    }

    #[allow(dead_code)]
    pub fn input_vector_row<T>(n: usize) -> Vec<T>
    where
        T: std::str::FromStr,
    {
        let mut v = Vec::with_capacity(n);
        for _ in 0..n {
            let buf = match input_line_str().parse() {
                Ok(r) => r,
                Err(_) => panic!("Parse Error"),
            };
            v.push(buf);
        }
        v
    }

    pub trait ToCharVec {
        fn to_charvec(&self) -> Vec<char>;
    }

    impl ToCharVec for String {
        fn to_charvec(&self) -> Vec<char> {
            self.to_string().chars().collect::<Vec<_>>()
        }
    }
}

use input::*;
use std::collections::VecDeque;

const TAKAHASHI: usize = 1;
const AOKI: usize = 2;

fn dfs(
    v: &Vec<Vec<usize>>,
    map: &Vec<usize>,
    k: usize,
    d: usize,
    parent: usize,
    ans: &mut Vec<usize>,
) {
    if map[k] == TAKAHASHI {
        ans.push(d);
    }
    for z in &v[k] {
        if *z == parent {
            continue;
        }
        dfs(v, map, *z, d + 1, k, ans);
    }
}

fn main() {
    let (n, u, w): (usize, usize, usize);
    input!(n, u, w);
    let mut v = vec![vec![]; n];
    let mut map = vec![0; n];
    for _ in 0..n - 1 {
        let (a, b): (usize, usize);
        input!(a, b);
        v[a - 1].push(b - 1);
        v[b - 1].push(a - 1);
    }

    let mut qt = VecDeque::new();
    let mut qa = VecDeque::new();

    qt.push_front(u - 1);
    map[u - 1] = TAKAHASHI;
    qa.push_front(w - 1);
    map[w - 1] = AOKI;

    while !qt.is_empty() || !qa.is_empty() {
        if !qt.is_empty() {
            let mut bufq = qt;
            qt = VecDeque::new();
            while !bufq.is_empty() {
                let buf = bufq.pop_back().unwrap();
                for z in &v[buf] {
                    if map[*z] == 0 {
                        map[*z] = TAKAHASHI;
                        qt.push_front(*z);
                    }
                }
            }
        }

        if !qa.is_empty() {
            let mut bufq = qa;
            qa = VecDeque::new();
            while !bufq.is_empty() {
                let buf = bufq.pop_back().unwrap();
                for z in &v[buf] {
                    if map[*z] == 0 {
                        map[*z] = AOKI;
                        qa.push_front(*z);
                    }
                }
            }
        }
    }
    let mut ans = vec![];
    dfs(&v, &map, w - 1, 0, w - 1, &mut ans);
    let z = *ans.iter().max().unwrap();
    p(z - 1);
}


Measured memory_bytes: 18940