//#pragma GCC optimize("Ofast,no-stack-protector")
//#pragma GCC target("sse,sse2,sse3,ssse3,sse4,popcnt,abm,mmx,avx,avx2")
/*#pragma GCC optimize("unroll-loops")
#pragma GCC optimize("fast-math")
#pragma GCC optimize("section-anchors")
#pragma GCC optimize("profile-values,profile-reorder-functions,tracer")
#pragma GCC optimize("vpt")
#pragma GCC optimize("rename-registers")
#pragma GCC optimize("move-loop-invariants")
#pragma GCC optimize("unswitch-loops")
#pragma comment(linker, "/STACK:36777216")
#pragma GCC optimize("function-sections")
#pragma GCC optimize("data-sections")
#pragma GCC optimize("branch-target-load-optimize")
#pragma GCC optimize("branch-target-load-optimize2")
#pragma GCC optimize("btr-bb-exclusive")*/
#define _CRT_SECURE_NO_WARNINGS
#include <chrono>
#include <set>
#include <map>
#include <deque>
#include <cmath>
#include <queue>
#include <cassert>
#include <random>
#include <bitset>
#include <iomanip>
#include <numeric>
#include <time.h>//////////////
#include <ctime>
#include <string>
#include <cstdio>
#include <vector>
#include <cstdlib>
#include <iostream>
#include <algorithm>
#include <unordered_map>
#include <unordered_set>
//++--++--++--++--++--++--++--++--++--++--++--++--++--++--++--++--++--++--++--++--++--++--++--++--++--++--++--++--++--++--++--++--++--++--++--++--++--++--++--++--++--++--++--++--++--++--++--++
//#define endl '\n'
#define mp make_pair
#define pbc push_back
#define pob pop_back()
#define empb emplace_back
#define queuel queue<long long>
#define sqr(a) ((a) * (a))
#define all(x) (x).begin(), (x).end()
#define rall(x) (x).rbegin(), (x).rend()
#define pin(p) cin >> p.first >> p.second;
#define uniq(a) sort(all(a));(a).resize(unique(all(a)) - a.begin());
#define rev(v) reverse(v.begin(), v.end());
#define sout(s, c) for (auto i : s) cout << i << c;
#define pout(p) cout << p.first << " " << p.second;
#define er(v, l, r) erase(v.begin() + l, v.begin() + r);
#define vin(v) for (ll i = 0; i < v.size(); ++i) cin >> v[i];
#define vout(v, c) for (int i = 0; i < v.size(); ++i) cout << v[i] << c;
#define pushi(v, a) for (int i = 0; i < a.size(); ++i) v.push_back(a[i]);
#define fastio() ios_base::sync_with_stdio(0); cout.tie(0); cin.tie(0); srand(time(NULL))
#define sp system("pause")
//++--++--++--++--++--++--++--++--++--++--++--++--++--++--++--++--++--++--++--++--++--++--++--++--++--++--++--++--++--++--++--++--++--++--++--++--++--++--++--++--++--++--++--++--++--++--++--++
using namespace std;
//++--++--++--++--++--++--++--++--++--++--++--++--++--++--++--++--++--++--++--++--++--++--++--++--++--++--++--++--++--++--++--++--++--++--++--++--++--++--++--++--++--++--++--++--++--++--++--++
typedef long long ll;
typedef long double ld;
typedef unsigned long long ull;
//++--++--++--++--++--++--++--++--++--++--++--++--++--++--++--++--++--++--++--++--++--++--++--++--++--++--++--++--++--++--++--++--++--++--++--++--++--++--++--++--++--++--++--++--++--++--++--++
const ld EPS = 1e-9;
const ld PI = acos(-1);
int mod = (int)998244353;
const int MOD7 = 1000000007;
const int MOD9 = 1000000009;
const int a228 = 18;
const ll kekmod = 1791791791;
const ll bestmod = 1148822869;
const ll secmod = (int) 1e9 + 113;
vector<ll> mods = { kekmod,bestmod,mod,MOD9,1000000007 };
vector<ll> hashpows = { 29,31,37,43,47,53,179,229 };
mt19937 rnd(chrono::high_resolution_clock::now().time_since_epoch().count() + 228 + 'i' + 'q' + 1337 + 1488);
//ll MOD = mods[rnd() % mods.size()];
//ll hashpow = hashpows[rand() % hashpows.size()];
const int MAXN = 2e5 + 1;
vector<vector<int>> g(MAXN);
vector<int> bfs(int v)
{
	vector<int> d(MAXN, 1e9);
	queue<int>q;
	q.push(v);
	d[v] = 0;
	while (q.size())
	{
		int u = q.front();
		q.pop();
		for (int i : g[u])
		{
			if (d[i] > d[u] + 1)
			{
				d[i] = d[u] + 1;
				q.push(i);
			}
		}
	}
	return d;
}
signed main()
{
	fastio();
	int x, y;
	int n;
	cin >> n >> x >> y;
	for (int i = 0; i < n - 1; ++i)
	{
		int a, b;
		cin >> a >> b;
		g[a].pbc(b);
		g[b].pbc(a);
	}
	vector<int> d1 = bfs(y);
	vector<int> d2(MAXN, 1e9);
	queue<int> q;
	d2[x] = 0;
	q.push(x);
	while (q.size())
	{
		int v = q.front();
		q.pop();
		if (d1[v] == d2[v])continue;
		for (int i : g[v])
		{
			if (d2[i] > d2[v] + 1)
			{
				d2[i] = d2[v] + 1;
				q.push(i);
			}
		}
	}
	int ans = 0;
	for (int i = 1; i <= n; ++i)
	{
		if (d1[i] == d2[i])
		{
			ans = max(ans, d1[i]);
		}
		if (d1[i] > d2[i])
		{
				ans = max(ans, d1[i] - 1);
		}
	}
	cout << ans;
//	sp;
}

Measured memory_bytes: 9856