package main

import (
	"bufio"
	"fmt"
	"os"
	"strconv"
)

func getScanner(fp *os.File) *bufio.Scanner {
	scanner := bufio.NewScanner(fp)
	scanner.Split(bufio.ScanWords)
	scanner.Buffer(make([]byte, 500001), 500000)
	return scanner
}

func getNextString(scanner *bufio.Scanner) string {
	scanner.Scan()
	return scanner.Text()
}

func getNextInt(scanner *bufio.Scanner) int {
	i, _ := strconv.Atoi(getNextString(scanner))
	return i
}

func getNextInt64(scanner *bufio.Scanner) int64 {
	i, _ := strconv.ParseInt(getNextString(scanner), 10, 64)
	return i
}

func getNextUint64(scanner *bufio.Scanner) uint64 {
	i, _ := strconv.ParseUint(getNextString(scanner), 10, 64)
	return i
}

func getNextFloat64(scanner *bufio.Scanner) float64 {
	i, _ := strconv.ParseFloat(getNextString(scanner), 64)
	return i
}

func main() {
	fp := os.Stdin
	wfp := os.Stdout

	if os.Getenv("MASPY") == "ますピ" {
		fp, _ = os.Open(os.Getenv("BEET_THE_HARMONY_OF_PERFECT"))
	}
	if os.Getenv("MASPYPY") == "ますピッピ" {
		wfp, _ = os.Create(os.Getenv("NGTKANA_IS_GENIUS10"))
	}

	scanner := getScanner(fp)
	writer := bufio.NewWriter(wfp)

	n := getNextInt(scanner)
	u := getNextInt(scanner) - 1
	v := getNextInt(scanner) - 1

	tr := Tree{}
	tr.init(n)
	for i := 0; i < n-1; i++ {
		a := getNextInt(scanner) - 1
		b := getNextInt(scanner) - 1
		tr.addEdge(a, Edge{
			to: b,
		})
		tr.addEdge(b, Edge{
			to: a,
		})
	}

	lca := Lca{}
	lca.init(n, u, tr.edges)

	ans := lca.depth[v] >> 1
	for i := 0; i < n; i++ {
		if i == u {
			continue
		}
		a := lca.get(i, v)
		if lca.depth[a] >= lca.depth[v]-lca.depth[a] {
			continue
		}
		d := lca.depth[v] - lca.depth[a] + lca.depth[i] - lca.depth[a] - 1
		if ans < d {
			ans = d
		}
	}

	fmt.Println(ans)

	writer.Flush()
}

// Tree ...
type Tree struct {
	n      int
	edges  [][]Edge
	dt, da []int
}

func (tr *Tree) init(n int) {
	edges := make([][]Edge, n)
	for i := 0; i < n; i++ {
		edges[i] = make([]Edge, 0)
	}
	tr.n = n
	tr.edges = edges
}

func (tr *Tree) addEdge(i int, e Edge) {
	tr.edges[i] = append(tr.edges[i], e)
}

// Edge ...
type Edge struct {
	to int
}

// Lca ...
type Lca struct {
	depth   []int
	parents [][]int
}

func (lca *Lca) init(n, r int, edges [][]Edge) {
	parents := make([][]int, 32)
	for i := 0; i < 32; i++ {
		parents[i] = make([]int, n)
	}
	lca.depth = make([]int, n)
	lca.parents = parents
	lca.dfs(r, -1, 0, edges)
	for i := 1; i < 32; i++ {
		for j := 0; j < n; j++ {
			lca.parents[i][j] = -1
			if lca.parents[i-1][j] == -1 {
				continue
			}
			lca.parents[i][j] = lca.parents[i-1][lca.parents[i-1][j]]
		}
	}
}

func (lca *Lca) dfs(i, p, h int, edges [][]Edge) {
	lca.depth[i] = h
	lca.parents[0][i] = p
	for _, e := range edges[i] {
		if e.to == p {
			continue
		}
		lca.dfs(e.to, i, h+1, edges)
	}
}

func (lca *Lca) get(u, v int) int {
	if lca.depth[u] > lca.depth[v] {
		u, v = v, u
	}
	h := lca.depth[v] - lca.depth[u]
	for i := 0; i < 32; i++ {
		if h>>uint(i)&1 == 1 {
			v = lca.parents[i][v]
		}
	}
	if u == v {
		return u
	}
	for i := 30; i >= 0; i-- {
		if lca.parents[i][u] == lca.parents[i][v] {
			continue
		}
		u = lca.parents[i][u]
		v = lca.parents[i][v]
	}
	return lca.parents[0][u]
}
