#![allow(unused_parens)]
#![allow(unused_imports)]
#![allow(non_upper_case_globals)]
#![allow(non_snake_case)]
#![allow(unused_mut)]
#![allow(unused_variables)]
#![allow(dead_code)]

#[allow(unused_macros)]
macro_rules! invec {
    ( $ t : ty ) => {{
        let mut s = String::new();
        match std::io::stdin().read_line(&mut s) {
            Ok(0) => Vec::<$t>::new(),
            Ok(n) => s
                .trim()
                .split_whitespace()
                .map(|s| s.parse::<$t>().unwrap())
                .collect::<Vec<$t>>(),
            Err(_) => Vec::<$t>::new(),
        }
    }};
}

#[allow(unused_macros)]
macro_rules! get {
      ($t:ty) => {
          {
              let mut line: String = String::new();
              std::io::stdin().read_line(&mut line).unwrap();
              line.trim().parse::<$t>().unwrap()
          }
      };
      ($($t:ty),*) => {
          {
              let mut line: String = String::new();
              std::io::stdin().read_line(&mut line).unwrap();
              let mut iter = line.split_whitespace();
              (
                  $(iter.next().unwrap().parse::<$t>().unwrap(),)*
              )
          }
      };
      ($t:ty; $n:expr) => {
          (0..$n).map(|_|
              get!($t)
          ).collect::<Vec<_>>()
      };
      ($($t:ty),*; $n:expr) => {
          (0..$n).map(|_|
              get!($($t),*)
          ).collect::<Vec<_>>()
      };
      ($t:ty ;;) => {
          {
              let mut line: String = String::new();
              std::io::stdin().read_line(&mut line).unwrap();
              line.split_whitespace()
                  .map(|t| t.parse::<$t>().unwrap())
                  .collect::<Vec<_>>()
          }
      };
      ($t:ty ;; $n:expr) => {
          (0..$n).map(|_| get!($t ;;)).collect::<Vec<_>>()
      };
}

#[allow(unused_macros)]
macro_rules! input {
    (source = $s:expr, $($r:tt)*) => {
        let mut iter = $s.split_whitespace();
        input_inner!{iter, $($r)*}
    };
    ($($r:tt)*) => {
        let mut s = {
            use std::io::Read;
            let mut s = String::new();
            std::io::stdin().read_to_string(&mut s).unwrap();
            s
        };
        let mut iter = s.split_whitespace();
        input_inner!{iter, $($r)*}
    };
}

macro_rules! input_inner {
    ($iter:expr) => {};
    ($iter:expr, ) => {};

    ($iter:expr, $var:ident : $t:tt $($r:tt)*) => {
        let $var = read_value!($iter, $t);
        input_inner!{$iter $($r)*}
    };
    ($iter:expr, mut $var:ident : $t:tt $($r:tt)*) => {
        let mut $var = read_value!($iter, $t);
        input_inner!{$iter $($r)*}
    };
}

#[allow(unused_macros)]
macro_rules! read_value {
    ($iter:expr, ( $($t:tt),* )) => {
        ( $(read_value!($iter, $t)),* )
    };

    ($iter:expr, [ $t:tt ; $len:expr ]) => {
        (0..$len).map(|_| read_value!($iter, $t)).collect::<Vec<_>>()
    };

    ($next:expr, [$t:tt]) => {
        {
            let len = read_value!($next, usize);
            (0..len).map(|_| read_value!($next, $t)).collect::<Vec<_>>()
        }
    };
    ($iter:expr, chars) => {
        read_value!($iter, String).chars().collect::<Vec<char>>()
    };

    ($iter:expr, usize1) => {
        read_value!($iter, usize) - 1
    };

    ($iter:expr, $t:ty) => {
        $iter.next().unwrap().parse::<$t>().expect("Parse error")
    };
}

#[allow(unused_macros)]
#[cfg(debug_assertions)]
macro_rules! mydbg {
    //($arg:expr) => (dbg!($arg))
    //($arg:expr) => (println!("{:?}",$arg));
      ($($a:expr),*) => {
          eprintln!(concat!($(stringify!($a), " = {:?}, "),*), $($a),*);
      }
}
#[cfg(not(debug_assertions))]
macro_rules! mydbg {
    ($($arg:expr),*) => {};
}

macro_rules! echo {
    ($($a:expr),*) => {
          $(println!("{}",$a))*
      }
}

struct SliceDisplay<'a, T: 'a>(&'a [T]);
impl<'a, T: std::fmt::Display + 'a> std::fmt::Display for SliceDisplay<'a, T> {
    fn fmt(&self, f: &mut std::fmt::Formatter) -> std::fmt::Result {
        for item in self.0 {
            write!(f, "{} ", item)?;
        }
        Ok(())
    }
}

use std::cmp::*;
use std::collections::*;
use std::ops::{Add, Div, Mul, Sub};

#[allow(dead_code)]
static INF_I64: i64 = 92233720368547758;
#[allow(dead_code)]
static INF_I32: i32 = 21474836;
#[allow(dead_code)]
static INF_USIZE: usize = 18446744073709551;
#[allow(dead_code)]
static M_O_D: usize = 1000000007;
#[allow(dead_code)]
static PAI: f64 = 3.1415926535897932;

trait IteratorExt: Iterator {
    fn toVec(self) -> Vec<Self::Item>;
}

impl<T: Iterator> IteratorExt for T {
    fn toVec(self) -> Vec<Self::Item> {
        self.collect()
    }
}

trait CharExt {
    fn toNum(&self) -> usize;
    fn toAlphabetIndex(&self) -> usize;
    fn toNumIndex(&self) -> usize;
}
impl CharExt for char {
    fn toNum(&self) -> usize {
        return *self as usize;
    }
    fn toAlphabetIndex(&self) -> usize {
        return self.toNum() - 'a' as usize;
    }
    fn toNumIndex(&self) -> usize {
        return self.toNum() - '0' as usize;
    }
}

trait VectorExt {
    fn joinToString(&self, s: &str) -> String;
}
impl<T: ToString> VectorExt for Vec<T> {
    fn joinToString(&self, s: &str) -> String {
        return self
            .iter()
            .map(|x| x.to_string())
            .collect::<Vec<_>>()
            .join(s);
    }
}

trait UsizeExt {
    fn pow(&self, n: usize) -> usize;
}
impl UsizeExt for usize {
    fn pow(&self, n: usize) -> usize {
        return ((*self as u64).pow(n as u32)) as usize;
    }
}

fn main() {
    input! {
       n: usize,
       u:usize1,
       v:usize1,
       edges:[(usize1,usize1);n-1],
    }

    let mut map = vec![vec![]; n];
    let mut used: Vec<Option<usize>> = vec![None; n];
    for &(f, t) in &edges {
        map[f].push(t);
        map[t].push(f);
    }

    let mut ans: usize = 0;
    let mut kouho = VecDeque::new();
    kouho.push_back((u, 0));
    used[u] = Some(INF_USIZE);
    let mut dist = 0;
    while !kouho.is_empty() {
        let (p, depth) = kouho.pop_front().unwrap();
        if p == v {
            break;
        }
        for i in 0..map[p].len() {
            let next = map[p][i];
            if used[next].is_some() {
                continue;
            }
            used[next] = Some(p);
            kouho.push_back((next, depth + 1));
        }
    }

    //mydbg!(dist, used);
    let mut path = vec![];
    path.push(v);
    let mut cur = v;
    loop {
        //mydbg!(cur);
        cur = used[cur].unwrap();
        if cur == INF_USIZE {
            break;
        }
        path.push(cur);
    }

    path.reverse();
    let b = path[min(path.len() - 1, ((path.len() + 1) / 2))];
    //mydbg!(path, b);
    let mut kouho = VecDeque::new();

    let mut canmove = vec![false; n];
    kouho.push_back((u, 0));
    canmove[u] = true;
    while !kouho.is_empty() {
        let (p, depth) = kouho.pop_front().unwrap();
        for i in 0..map[p].len() {
            let next = map[p][i];
            if canmove[next] {
                continue;
            }
            if next == b {
                continue;
            }
            canmove[next] = true;
            kouho.push_back((next, depth + 1));
        }
    }

    //mydbg!(canmove);
    let mut kouho = VecDeque::new();
    let mut used2: Vec<Option<usize>> = vec![None; n];
    kouho.push_back((v, 0));
    used2[v] = Some(0);
    while !kouho.is_empty() {
        let (p, depth) = kouho.pop_front().unwrap();
        for i in 0..map[p].len() {
            let next = map[p][i];
            if used2[next].is_some() {
                continue;
            }
            used2[next] = Some(depth + 1);
            kouho.push_back((next, depth + 1));
        }
    }
    //mydbg!(used2);

    let mut tmp = used2
        .iter()
        .map(|x| if x.is_some() { x.unwrap() } else { 0 })
        .zip(canmove)
        .filter(|&(_, x)| x)
        .toVec();

    tmp.sort_by_key(|x| x.0);
    tmp.reverse();
    //mydbg!(tmp);
    ans = tmp[0].0.saturating_sub(1);
    echo!(ans);
}


Measured memory_bytes: 26792