Problem:
Score : 600 points

Problem Statement

We have a tree with N vertices. The i-th edge connects Vertex A_i and B_i bidirectionally.

Takahashi is standing at Vertex u, and Aoki is standing at Vertex v.

Now, they will play a game of tag as follows:

1. If Takahashi and Aoki are standing at the same vertex, the game ends. Otherwise, Takahashi moves to a vertex of his choice that is adjacent to his current vertex.

2. If Takahashi and Aoki are standing at the same vertex, the game ends. Otherwise, Aoki moves to a vertex of his choice that is adjacent to his current vertex.

3. Go back to step 1.

Takahashi performs his moves so that the game ends as late as possible, while Aoki performs his moves so that the game ends as early as possible.

Find the number of moves Aoki will perform before the end of the game if both Takahashi and Aoki know each other's position and strategy.

It can be proved that the game is bound to end.

Constraints

2 \leq N \leq 10^5

1 \leq u,v \leq N

u \neq v

1 \leq A_i,B_i \leq N

The given graph is a tree.

Input

Input is given from Standard Input in the following format:

N u v
A_1 B_1
:
A_{N-1} B_{N-1}

Output

Print the number of moves Aoki will perform before the end of the game.

Sample Input 1

5 4 1
1 2
2 3
3 4
3 5

Sample Output 1

2

If both players play optimally, the game will progress as follows:

Takahashi moves to Vertex 3.

Aoki moves to Vertex 2.

Takahashi moves to Vertex 5.

Aoki moves to Vertex 3.

Takahashi moves to Vertex 3.

Here, Aoki performs two moves.

Note that, in each move, it is prohibited to stay at the current vertex.

Sample Input 2

5 4 5
1 2
1 3
1 4
1 5

Sample Output 2

1

Sample Input 3

2 1 2
1 2

Sample Output 3

0

Sample Input 4

9 6 1
1 2
2 3
3 4
4 5
5 6
4 7
7 8
8 9

Sample Output 4

5

Solution in Rust (1.42.0) (judge verdict: Accepted)

// -*- coding:utf-8-unix -*-

extern crate lazy_static;
extern crate num_bigint;
// 0.2.2
extern crate num_traits;

// 0.2.8
use num_bigint::BigInt;
use num_traits::{one, zero, Num, NumAssignOps, NumOps, One, Pow, Zero};

// use proconio::derive_readable;
use proconio::fastout;
use proconio::input;
use proconio::marker::Chars;
// use std::convert::TryInto;
use itertools::{assert_equal, concat, Itertools};
use lazy_static::lazy_static;
// use libm::*;
use std::cmp::*;
use std::collections::{BinaryHeap, HashMap, HashSet, VecDeque};
use std::io::*;
use std::mem::swap;
use std::ops::{BitAnd, Range, ShrAssign, Neg};
use std::str::FromStr;
use std::sync::Mutex;
use superslice::*;

// ##########
// read
// ###########
pub fn read<T: FromStr>() -> T {
    let stdin = stdin();
    let stdin = stdin.lock();
    let token: String = stdin
        .bytes()
        .map(|c| c.expect("failed to read char") as char)
        .skip_while(|c| c.is_whitespace())
        .take_while(|c| !c.is_whitespace())
        .collect();
    token.parse().ok().expect("failed to parse token")
}

// ##########
// chmin, chmax
// https://qiita.com/maguro_tuna/items/fab200fdc1efde1612e7
// ###########

#[allow(unused_macros)]
macro_rules! chmin {
    ($base:expr, $($cmps:expr),+ $(,)*) => {{
        let cmp_min = min!($($cmps),+);
        if $base > cmp_min {
            $base = cmp_min;
            true
        } else {
            false
        }
    }};
}

#[allow(unused_macros)]
macro_rules! chmax {
    ($base:expr, $($cmps:expr),+ $(,)*) => {{
        let cmp_max = max!($($cmps),+);
        if $base < cmp_max {
            $base = cmp_max;
            true
        } else {
            false
        }
    }};
}

#[allow(unused_macros)]
macro_rules! min {
    ($a:expr $(,)*) => {{
        $a
    }};
    ($a:expr, $b:expr $(,)*) => {{
        std::cmp::min($a, $b)
    }};
    ($a:expr, $($rest:expr),+ $(,)*) => {{
        std::cmp::min($a, min!($($rest),+))
    }};
}

#[allow(unused_macros)]
macro_rules! max {
    ($a:expr $(,)*) => {{
        $a
    }};
    ($a:expr, $b:expr $(,)*) => {{
        std::cmp::max($a, $b)
    }};
    ($a:expr, $($rest:expr),+ $(,)*) => {{
        std::cmp::max($a, max!($($rest),+))
    }};
}



// ##########
// modint
// https://qiita.com/drken/items/3b4fdf0a78e7a138cd9a
// ##########

#[allow(dead_code)]
fn modinv<T: Num + NumAssignOps + NumOps + Copy + PartialOrd + Neg>(a: T, m: T) -> T {
    let mut a = a;
    let mut b = m;
    let mut u: T = one();
    let mut v: T = zero();

    while b != zero() {
        let t = a / b;
        a -= t * b;
        swap(&mut a, &mut b);
        u -= t * v;
        swap(&mut u, &mut v);
    }
    u %= m;
    if u < zero() {
        u += m;
    }
    return u;
}

#[test]
fn modinv_test() {
    assert_eq!(1, modinv(1, 13));
    assert_eq!(2, modinv(7, 13));
    assert_eq!(3, modinv(9, 13));
    assert_eq!(4, modinv(10, 13));
    assert_eq!(5, modinv(8, 13));
}

// long long modpow(long long a, long long n, long long mod) {
// long long res = 1;
// while (n > 0) {
// if (n & 1) res = res * a % mod;
// a = a * a % mod;
// n >>= 1;
// }
// return res;
// }
#[allow(dead_code)]
fn modpow<T>(a: T, n: T, modulo: T) -> T
where
    T: Num + NumAssignOps + NumOps + Copy + PartialOrd + BitAnd + PartialEq + ShrAssign,
    <T as BitAnd>::Output: PartialEq + Num,
{
    let mut res = one();
    let mut a = a;
    let mut n = n;
    while n > zero() {
        if (n & one()) == one() {
            res = res * a % modulo;
        }
        a = a * a % modulo;
        n >>= one();
    }

    return res;
}

#[test]
fn modpow_test() {
    assert_eq!(4, modpow(2, 2, 13));
    assert_eq!(3, modpow(2, 4, 13));
}

// 前処理 com_init(): O(n)
// クエリ処理 COM(n, k): O(1)
// conv::com_init();
// conv::com(n,k);
mod comb {
    use super::*;
    lazy_static! {
        static ref FAC: Mutex<Vec<usize>> = Mutex::default();
        static ref FINV: Mutex<Vec<usize>> = Mutex::default();
        static ref INV: Mutex<Vec<usize>> = Mutex::default();
        static ref MODULO: Mutex<usize> = Mutex::default();
        // static ref MAXNCONV: Mutex<usize> = Mutex::default();
    }

    // // テーブルを作る前処理
    // void com_init() {
    // fac[0] = fac[1] = 1;
    // finv[0] = finv[1] = 1;
    // inv[1] = 1;
    // for (int i = 2; i < MAX; i++){
    // fac[i] = fac[i - 1] * i % MOD;
    // inv[i] = MOD - inv[MOD%i] * (MOD / i) % MOD;
    // finv[i] = finv[i - 1] * inv[i] % MOD;
    // }

    #[allow(dead_code)]
    fn com_init_with(modulo: usize, maxn_conv: usize) {
        let mut fac = FAC.lock().unwrap();
        let mut finv = FINV.lock().unwrap();
        let mut inv = INV.lock().unwrap();
        *fac = vec![0; maxn_conv];
        *finv = vec![0; maxn_conv];
        *inv = vec![0; maxn_conv];

        let mut g_modulo = MODULO.lock().unwrap();
        *g_modulo = modulo;

        fac[0] = 1;
        fac[1] = 1;
        finv[0] = 1;
        finv[1] = 1;
        inv[1] = 1;

        for i in 2..maxn_conv {
            fac[i] = fac[i - 1] * i % modulo;
            inv[i] = modulo - inv[modulo % i] * (modulo / i) % modulo;
            finv[i] = finv[i - 1] * inv[i] % modulo;
        }
    }

    #[allow(dead_code)]
    pub fn com_init() {
        com_init_with(MOD, MAXN_CONV);
    }

    // // 二項係数計算
    // long long COM(int n, int k){
    // if (n < k) return 0;
    // if (n < 0 || k < 0) return 0;
    // return fac[n] * (finv[k] * finv[n - k] % MOD) % MOD;
    // }
    #[allow(dead_code)]
    pub fn com(n: usize, k: usize) -> usize {
        let fac = FAC.lock().unwrap();
        let finv = FINV.lock().unwrap();
        // let mut inv = INV.lock().unwrap();
        let m = *MODULO.lock().unwrap();

        if n < k {
            return 0;
        }
        // if n < 0 || k < 0 {
        //     return 0;
        // }
        return fac[n] * (finv[k] * finv[n - k] % m) % m;
    }

    #[test]
    fn com_test() {
        com_init_with(13, 100);
        assert_eq!(12, com(12, 1));
        assert_eq!(66 % 13, com(12, 2));
        assert_eq!(220 % 13, com(12, 3));
        assert_eq!(495 % 13, com(12, 4));
        assert_eq!(792 % 13, com(12, 5));
        assert_eq!(924 % 13, com(12, 6));
        assert_eq!(com(12, 5), com(12, 7));
    }
}

// ##########
// union-find
// http://sntea.hatenablog.com/entry/2017/06/07/091246
// ##########

mod uf {
    // let mut uf = uf::UnionFind::new(10);
    // uf.unite; uf.same
    #[allow(dead_code)]
    #[derive(Debug)]
    pub struct UnionFind {
    par: Vec<i64>,
        rank: Vec<usize>,
    }

    impl UnionFind {
        #[allow(dead_code)]
        pub fn new(n: usize) -> UnionFind {
            let mut vec = vec![0; n];
            for i in 0..n {
                vec[i] = -1;
            }
            UnionFind {
                par: vec,
                rank: vec![0; n],
            }
        }

        #[allow(dead_code)]
        fn find(&mut self, x: usize) -> usize {
            if self.par[x] < 0 {
                x
            } else {
                let par = self.par[x];
                let res = self.find(par as usize);
                self.par[x] = res as i64;
                res
            }
        }

        #[allow(dead_code)]
        pub fn same(&mut self, a: usize, b: usize) -> bool {
            self.find(a) == self.find(b)
        }

        #[allow(dead_code)]
        pub fn unite(&mut self, a: usize, b: usize) {
            let apar = self.find(a);
            let bpar = self.find(b);
            if self.rank[apar] > self.rank[bpar] {
                self.par[apar] += self.par[bpar];
                self.par[bpar] = apar as i64;
            } else {
                self.par[bpar] += self.par[apar];
                self.par[apar] = bpar as i64;
                if self.rank[apar] == self.rank[bpar] {
                    self.rank[bpar] += 1;
                }
            }
        }

        #[allow(dead_code)]
        pub fn size(&mut self, x: usize) -> usize {
            let parent = self.find(x);
            //parentのparにサイズが負の状態で入る
            return (-self.par[parent]) as usize;
        }
    }

    #[test]
    fn union_find_test() {
        let mut uf = UnionFind::new(10);

        for i in 0..10 {
            for j in 0..10 {
                assert_eq!(i == j, uf.same(i, j));
            }
        }

        uf.unite(0, 1);
        assert_eq!(true, uf.same(0, 1));
        //false
        assert_eq!(false, uf.same(0, 9));
        assert_eq!(false, uf.same(1, 9));
        assert_eq!(false, uf.same(2, 9));

        assert_eq!(2, uf.size(0));
        assert_eq!(2, uf.size(1));
        //1
        assert_eq!(1, uf.size(2));
        assert_eq!(1, uf.size(8));
        assert_eq!(1, uf.size(9));

        uf.unite(8, 9);
        assert_eq!(true, uf.same(0, 1));
        assert_eq!(true, uf.same(8, 9));
        //false
        assert_eq!(false, uf.same(0, 9));
        assert_eq!(false, uf.same(1, 9));
        assert_eq!(false, uf.same(2, 9));

        assert_eq!(2, uf.size(0));
        assert_eq!(2, uf.size(1));
        assert_eq!(2, uf.size(8));
        assert_eq!(2, uf.size(9));
        //1
        assert_eq!(1, uf.size(2));

        uf.unite(1, 9);
        assert_eq!(true, uf.same(0, 1));
        assert_eq!(true, uf.same(8, 9));
        assert_eq!(true, uf.same(0, 8));
        assert_eq!(true, uf.same(0, 9));
        assert_eq!(true, uf.same(1, 8));
        assert_eq!(true, uf.same(1, 9));
        //false
        assert_eq!(false, uf.same(2, 9));

        assert_eq!(4, uf.size(0));
        assert_eq!(4, uf.size(1));
        assert_eq!(4, uf.size(8));
        assert_eq!(4, uf.size(9));
        //1
        assert_eq!(1, uf.size(2));
    }
}

// ###########
// seg_tree
// ant_book
// ###########
mod seg_tree {
    #[derive(Debug)]
    pub struct SegTree<T: Clone> {
        n: usize,
        dat: Vec<Option<T>>,
    }

    impl<T: Clone + std::fmt::Debug> SegTree<T> {
        #[allow(dead_code)]
        pub fn new(size: usize) -> SegTree<T> {
            let mut size_pow2 = 1;
            while size_pow2 < size {
                size_pow2 *= 2;
            }

            let dat: Vec<Option<T>> = vec![None; 2 * size_pow2 - 1];
            SegTree { n: size_pow2, dat }
        }

        #[allow(dead_code)]
        pub fn update<F: Fn(&Option<T>, &Option<T>) -> Option<T>>(
            &mut self,
            k: usize,
            a: T,
            update: F,
        ) {
            let mut k = k;
            k += self.n - 1;
            self.dat[k] = Some(a);
            while k > 0 {
                k = (k - 1) / 2;
                self.dat[k] = update(&self.dat[k * 2 + 1], &self.dat[k * 2 + 2]);
            }
        }

        #[allow(dead_code)]
        fn query_inner<F: Fn(&Option<T>, &Option<T>) -> Option<T>>(
            &self,
            selection_query: &F,
            a: usize,
            b: usize,
            k: usize,
            l: usize,
            r: usize,
        ) -> Option<T> {
            if r <= a || b <= l {
                // eprintln!("{}, {}, {}, {}, {:?}", a, b, l, r, "none");
                return None;
            }

            return if a <= l && r <= b {
                // eprintln!("{}, {}, {}, {}, {:?}", a, b, r, l, self.dat[k]);
                self.dat[k].clone()
            } else {
                let vl = self.query_inner(selection_query, a, b, k * 2 + 1, l, (l + r) / 2);
                let vr = self.query_inner(selection_query, a, b, k * 2 + 2, (l + r) / 2, r);

                selection_query(&vl, &vr)
            };
        }

        #[allow(dead_code)]
        pub fn query<F: Fn(&Option<T>, &Option<T>) -> Option<T>>(
            &self,
            selection_query: &F,
            a: usize,
            b: usize,
        ) -> Option<T> {
            return self.query_inner(selection_query, a, b, 0, 0, self.n);
        }
    }

    #[test]
    fn test_segtree_rmq() {
        let mut t: SegTree<usize> = SegTree::new(5);

        let cmp_f = |lhs: &Option<usize>, rhs: &Option<usize>| {
            if lhs.is_none() {
                return rhs.clone();
            }

            if rhs.is_none() {
                return lhs.clone();
            }

            return if lhs.unwrap() <= rhs.unwrap() {
                lhs.clone()
            } else {
                rhs.clone()
            };
        };
        // 1, 3, 2, 5, 1
        t.update(0, 1, cmp_f);
        t.update(1, 3, cmp_f);
        t.update(2, 2, cmp_f);
        t.update(3, 5, cmp_f);
        t.update(4, 1, cmp_f);
        // println!("{:?}", t);

        assert_eq!(1, t.query(&cmp_f, 0, 1).unwrap());
        assert_eq!(3, t.query(&cmp_f, 1, 2).unwrap());
        assert_eq!(2, t.query(&cmp_f, 2, 3).unwrap());
        assert_eq!(5, t.query(&cmp_f, 3, 4).unwrap());
        assert_eq!(1, t.query(&cmp_f, 4, 5).unwrap());

        // len2
        assert_eq!(1, t.query(&cmp_f, 0, 2).unwrap());
        assert_eq!(2, t.query(&cmp_f, 1, 3).unwrap());
        assert_eq!(2, t.query(&cmp_f, 2, 4).unwrap());
        assert_eq!(1, t.query(&cmp_f, 3, 5).unwrap());

        // len3
        assert_eq!(1, t.query(&cmp_f, 0, 3).unwrap());
        assert_eq!(2, t.query(&cmp_f, 1, 4).unwrap());
        assert_eq!(1, t.query(&cmp_f, 2, 5).unwrap());

        // len4
        assert_eq!(1, t.query(&cmp_f, 0, 4).unwrap());
        assert_eq!(1, t.query(&cmp_f, 1, 5).unwrap());

        // len5
        assert_eq!(1, t.query(&cmp_f, 0, 6).unwrap());
    }
}

// ##############
// rolling hash
// ###############
mod rolling_hash {
    use super::*;
    use ascii::{AsciiStr, AsciiString};
    use num_traits::AsPrimitive;

    fn contains_with(base: u64, a: &AsciiStr, b: &AsciiStr) -> bool {
        let (al, bl) = (a.len(), b.len());
        if al > bl {
            return false;
        }

        let mut t: u64 = 1;
        for _ in 0..al {
            t = t.wrapping_mul(base);
        }

        let (mut ah, mut bh): (u64, u64) = (0, 0);

        for i in 0..al {
            ah = ah.wrapping_mul(base) + a[i].as_byte() as u64;
        }
        for i in 0..al {
            bh = bh.wrapping_mul(base) + b[i].as_byte() as u64;
        }

        // eprintln!("{}, {}", ah, bh);

        for i in 0..=bl - al {
            if ah == bh {
                return true;
            }
            if i + al < bl {
                let mut add: i64 = b[i + al].as_byte().as_();
                add -= ((b[i].as_byte() as u64).wrapping_mul(t)) as i64;
                bh = (bh.wrapping_mul(base) as i64).wrapping_add(add) as u64;
            }
        }

        return false;
    }

    #[allow(dead_code)]
    pub fn contains(a: &AsciiStr, b: &AsciiStr) -> bool {
        return contains_with(BASE_ROLLING_HASH, a, b);
    }

    #[test]
    fn contains_test() {
        const base: u64 = 1000_000_007;
        assert_eq!(
            false,
            contains_with(
                base,
                &AsciiString::from_str("abc").unwrap(),
                &AsciiString::from_str("a").unwrap()
            )
        );

        assert_eq!(
            true,
            contains_with(
                base,
                &AsciiString::from_str("abc").unwrap(),
                &AsciiString::from_str("aaabca").unwrap()
            )
        );

        assert_eq!(
            true,
            contains_with(
                base,
                &AsciiString::from_str("aaaaaa").unwrap(),
                &AsciiString::from_str("aaaaaa").unwrap()
            )
        );

        assert_eq!(
            false,
            contains_with(
                base,
                &AsciiString::from_str("abc").unwrap(),
                &AsciiString::from_str("aacbaa").unwrap()
            )
        )
    }

    fn overlap_last_and_head_with(base: u64, a: &AsciiStr, b: &AsciiStr) -> usize {
        let (al, bl) = (a.len(), b.len());

        let mut ans = 0;
        let (mut ah, mut bh, mut t): (u64, u64, u64) = (0, 0, 1);
        for i in 1..=min(al, bl) {
            ah = ah.wrapping_add((a[al - i].as_byte() as u64).wrapping_mul(t));
            bh = bh
                .wrapping_mul(base)
                .wrapping_add(b[i - 1].as_byte() as u64);

            if ah == bh {
                ans = i;
            }
            t = t.wrapping_mul(base);
        }

        return ans;
    }

    fn overlap_last_and_head(a: &AsciiStr, b: &AsciiStr) -> usize {
        return overlap_last_and_head_with(BASE_ROLLING_HASH, a, b);
    }

    #[test]
    fn overlap_test() {
        const base: u64 = 1000_000_007;
        assert_eq!(
            0,
            overlap_last_and_head_with(
                base,
                &AsciiString::from_str("abc").unwrap(),
                &AsciiString::from_str("a").unwrap()
            )
        );

        assert_eq!(
            2,
            overlap_last_and_head_with(
                base,
                &AsciiString::from_str("abc").unwrap(),
                &AsciiString::from_str("bca").unwrap()
            )
        );

        assert_eq!(
            5,
            overlap_last_and_head_with(
                base,
                &AsciiString::from_str("hogefoobar").unwrap(),
                &AsciiString::from_str("oobarhoge").unwrap()
            )
        );
    }
}

// ##########
// lazy_static!
// ##########
// lazy_static! {
//     static ref H: Mutex<Vec<i32>> = Mutex::default();
//     static ref W: Mutex<Vec<i32>> = Mutex::default();
// }
// let mut values = VALUES.lock().unwrap();
// values.extend_from_slice(&[1, 2, 3, 4]);
// assert_eq!(&*values, &[1, 2, 3, 4]);

// let mut values = VALUES.lock().unwrap();
// values.extend_from_slice(&[1, 2, 3, 4]);

// MOD, Combination関連に使う定数
#[allow(dead_code)]
const BASE_ROLLING_HASH: u64 = 1158187049;
#[allow(dead_code)]
const MOD: usize = 1000000007;
#[allow(dead_code)]
const MAXN_CONV: usize = 510000;

#[allow(dead_code)]
const INF: usize = 10_000_000_000;


// abc148-F
// #[fastout]
fn main() {
    input![n: usize, u:usize, v:usize,
        ab: [(usize, usize); n-1]];
    //new type
    let ab = ab.iter().map(|(a,b)| (a-1, b-1)).collect_vec();
    let tmp = u;
    let u = v-1;
    let v = tmp-1;

    let mut adj = vec![vec![]; n];
    for (a,b) in ab{
        adj[a].push(b);
        adj[b].push(a);
    }

    let mut diff2u = vec![INF; n];
    diff2u[u] = 0;

    let mut q = VecDeque::new();
    q.push_back((u, diff2u[u]));

    while !q.is_empty(){
        let (s, d) = q.pop_front().unwrap();
        for to in adj[s].iter(){
            if diff2u[*to] != INF {
                continue;
            }

            diff2u[*to] = d + 1;
            q.push_back((*to, diff2u[*to]));
        }
    }

    let mut diff2v = vec![INF; n];
    // let mut diff2v2 = vec![INF; n];

    diff2v[v] = 0;
    let mut q = VecDeque::new();
    q.push_back((v, diff2v[v]));

    let mut ans: isize = -1;

    if adj[v].len() == 1{
        // eprintln!("ans initted: {}",  diff2u[v] / 2);
        ans = diff2u[v] as isize / 2 ;
    }

    while !q.is_empty(){
        let (s, d) = q.pop_front().unwrap();

        let mut is_leaf = true;
        for to in adj[s].iter(){
            if diff2v[*to] != INF {
                continue;
            }
            is_leaf = false;

            diff2v[*to] = d + 1;

            if diff2u[*to] <= diff2v[*to]{
                continue;
            }

            q.push_back((*to, diff2v[*to]));
        }

        if adj[s].len() == 1{
            // eprintln!()
            // eprintln!("s: {}, {}, {}", s, ans, diff2u[s]);
            chmax!(ans, diff2u[s] as isize -1); // / 2 * 2);
        }
    }

    // eprintln!("{:?}", diff2u);
    // eprintln!("{:?}", diff2v);

    println!("{}", ans);


}