fn construct_paths(
    pos: usize,
    pre: Option<usize>,
    edges: &Vec<Vec<usize>>,
    paths: &mut Vec<usize>,
    u: usize,
    v: usize,
) -> bool {
    if pos == v {
        return true;
    }
    paths.push(pos);
    for &nxt in edges[pos].iter() {
        if let Some(p) = pre {
            if nxt == p {
                continue;
            }
        }
        if construct_paths(nxt, Some(pos), edges, paths, u, v) {
            return true;
        }
    }
    paths.pop();
    false
}

fn dfs(
    pos: usize,
    pre: Option<usize>,
    edges: &Vec<Vec<usize>>,
    cnt: usize,
    costs: &mut Vec<usize>,
    v: usize,
) {
    if pos == v {
        return;
    }
    costs[pos] = cnt;
    for &nxt in edges[pos].iter() {
        if let Some(p) = pre {
            if p == nxt {
                continue;
            }
        }
        dfs(nxt, Some(pos), edges, cnt + 1, costs, v);
    }
}

fn solve() {
    let stdin = std::io::stdin();
    let mut scn = Scanner {
        stdin: stdin.lock(),
    };
    let n: usize = scn.read();
    let u: usize = scn.read::<usize>() - 1;
    let v: usize = scn.read::<usize>() - 1;
    let mut edges = vec![vec![]; n];
    let ab: Vec<(usize, usize)> = (0..n - 1)
        .map(|_| {
            let a: usize = scn.read::<usize>() - 1;
            let b: usize = scn.read::<usize>() - 1;
            edges[a].push(b);
            edges[b].push(a);
            (a, b)
        })
        .collect();
    let mut paths = vec![];
    construct_paths(u, None, &edges, &mut paths, u, v);

    let mut result = 0;
    let d = paths.len() / 2;

    let (u, v) = if paths.len() >= 3 {
        result += d;
        let node = paths[d];
        let u = node;
        let v = if d + 1 < paths.len() { paths[d + 1] } else { v };
        (u, v)
    } else {
        (paths[d], v)
    };

    //let u = node;
    //let v = if paths.len() >= 3 && d + 1 < paths.len() { paths[d + 1] } else { v };
    //eprintln!("{:?} {:?}", u, v);
    //eprintln!("{:?}", paths);
    //eprintln!("{:?}", node);

    let mut costs = vec![0; n];
    dfs(u, None, &edges, 0, &mut costs, v);
    ///eprintln!("{:?}", costs);
    let d1 = costs.iter().max().unwrap();

    println!("{}", result + d1);
}

fn main() {
    std::thread::Builder::new()
        .stack_size(64 * 1024 * 1024) // 64MB
        .spawn(|| solve())
        .unwrap()
        .join()
        .unwrap();
}
pub mod utils {
    const DYX: [(isize, isize); 8] = [
        (0, 1),   //right
        (1, 0),   //down
        (0, -1),  //left
        (-1, 0),  //top
        (1, 1),   //down right
        (-1, 1),  //top right
        (1, -1),  //down left
        (-1, -1), //top left
    ];

    pub fn try_adj(y: usize, x: usize, dir: usize, h: usize, w: usize) -> Option<(usize, usize)> {
        let ny = y as isize + DYX[dir].0;
        let nx = x as isize + DYX[dir].1;
        if ny >= 0 && nx >= 0 {
            let ny = ny as usize;
            let nx = nx as usize;
            if ny < h && nx < w {
                Some((ny, nx))
            } else {
                None
            }
        } else {
            None
        }
    }
}

//snippet from kenkoooo
pub struct Scanner<R> {
    stdin: R,
}

impl<R: std::io::Read> Scanner<R> {
    pub fn read<T: std::str::FromStr>(&mut self) -> T {
        use std::io::Read;
        let buf = self
            .stdin
            .by_ref()
            .bytes()
            .map(|b| b.unwrap())
            .skip_while(|&b| b == b' ' || b == b'\n' || b == b'\r')
            .take_while(|&b| b != b' ' && b != b'\n' && b != b'\r')
            .collect::<Vec<_>>();
        unsafe { std::str::from_utf8_unchecked(&buf) }
            .parse()
            .ok()
            .expect("Parse error.")
    }
    pub fn read_line(&mut self) -> String {
        use std::io::Read;
        let buf = self
            .stdin
            .by_ref()
            .bytes()
            .map(|b| b.unwrap())
            .skip_while(|&b| b == b'\n' || b == b'\r')
            .take_while(|&b| b != b'\n' && b != b'\r')
            .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()
    }
}
