#include <bits/stdc++.h>
using namespace std;
#if __has_include(<boost/multiprecision/cpp_int.hpp>)
	#include <boost/multiprecision/cpp_int.hpp>
	#include <boost/multiprecision/cpp_dec_float.hpp>
	using bll = boost::multiprecision::cpp_int;
	using bdouble = boost::multiprecision::cpp_dec_float_100;
	using namespace boost::multiprecision;
#endif
#if __has_include(<boost/stacktrace.hpp>)
	#define BOOST_STACKTRACE_USE_ADDR2LINE
	#define BOOST_STACKTRACE_ADDR2LINE_LOCATION /usr/local/opt/binutils/bin/addr2line
	#define _GNU_SOURCE 1
	#include <boost/stacktrace.hpp>
#endif
#ifdef LOCAL_TEST
	namespace std {
		template<typename T> class dvector : public std::vector<T> {
		public:
			dvector() : std::vector<T>() {}
			explicit dvector(size_t n, const T& value = T()) : std::vector<T>(n, value) {}
			dvector(const std::vector<T>& v) : std::vector<T>(v) {}
			dvector(const std::initializer_list<T> il) : std::vector<T>(il) {}
			dvector(const std::string::iterator first, const std::string::iterator last) : std::vector<T>(first, last) {}
			dvector(const typename std::vector<T>::iterator first, const typename std::vector<T>::iterator last) : std::vector<T>(first, last) {}
			dvector(const typename std::vector<T>::reverse_iterator first, const typename std::vector<T>::reverse_iterator last) : std::vector<T>(first, last) {}
			dvector(const typename std::vector<T>::const_iterator first, const typename std::vector<T>::const_iterator last) : std::vector<T>(first, last) {}
			dvector(const typename std::vector<T>::const_reverse_iterator first, const typename std::vector<T>::const_reverse_iterator last) : std::vector<T>(first, last) {}
			T& operator[](size_t n) {
				try { return this->at(n); } catch (const std::exception& e) { std::cerr << boost::stacktrace::stacktrace() << '\n'; return this->at(n); }
			}
			const T& operator[](size_t n) const {
				try { return this->at(n); } catch (const std::exception& e) { std::cerr << boost::stacktrace::stacktrace() << '\n'; return this->at(n); }
			}
		};
	}
	class dbool {
	private:
		bool boolvalue;
	public:
		dbool() : boolvalue(false) {}
		dbool(bool b) : boolvalue(b) {}
		operator bool&() { return boolvalue; }
		operator const bool&() const { return boolvalue; }
	};
	#define vector dvector
	#define bool dbool
#endif
#ifdef LOCAL_DEV
	template<typename T> std::ostream& operator<<(std::ostream& s, const std::vector<T>& v) {
		for (size_t i = 0; i < v.size(); ++i){ s << v[i]; if (i < v.size() - 1) s << "\t"; } return s; }
	template<typename T> std::ostream& operator<<(std::ostream& s, const std::vector<std::vector<T>>& vv) {
		s << "\\\n"; for (size_t i = 0; i < vv.size(); ++i){ s << vv[i] << "\n"; } return s; }
	template<typename T> std::ostream& operator<<(std::ostream& s, const std::deque<T>& v) {
		for (size_t i = 0; i < v.size(); ++i){ s << v[i]; if (i < v.size() - 1) s << "\t"; } return s; }
	template<typename T> std::ostream& operator<<(std::ostream& s, const std::set<T>& se) {
		s << "{ "; for (auto itr = se.begin(); itr != se.end(); ++itr){ s << (*itr) << "\t"; } s << "}"; return s; }
	template<typename T> std::ostream& operator<<(std::ostream& s, const std::multiset<T>& se) {
		s << "{ "; for (auto itr = se.begin(); itr != se.end(); ++itr){ s << (*itr) << "\t"; } s << "}"; return s; }
	template <typename T, size_t N> std::ostream& operator<<(std::ostream& s, const std::array<T, N>& a) {
		s << "{ "; for (size_t i = 0; i < N; ++i){ s << a[i] << "\t"; } s << "}"; return s; }
	template<typename T1, typename T2> std::ostream& operator<<(std::ostream& s, const std::map<T1, T2>& m) {
		s << "{\n"; for (auto itr = m.begin(); itr != m.end(); ++itr){ s << "\t" << (*itr).first << " : " << (*itr).second << "\n"; } s << "}"; return s; }
	template<typename T1, typename T2> std::ostream& operator<<(std::ostream& s, const std::pair<T1, T2>& p) {
		return s << "(" << p.first << ", " << p.second << ")"; }
	class SIGFPE_exception : std::exception {};
	class SIGSEGV_exception : std::exception {};
	void catch_SIGFPE([[maybe_unused]] int e) { std::cerr << boost::stacktrace::stacktrace() << '\n'; throw SIGFPE_exception(); }
	void catch_SIGSEGV([[maybe_unused]] int e) { std::cerr << boost::stacktrace::stacktrace() << '\n'; throw SIGSEGV_exception(); }
	signed convertedmain();
	signed main() { signal(SIGFPE, catch_SIGFPE); signal(SIGSEGV, catch_SIGSEGV); return convertedmain(); }
	#define main() convertedmain()
	void debug_impl() { std::cerr << '\n'; }
	template<typename Head, typename... Tail> void debug_impl(Head head, Tail... tail) { std::cerr << " " << head << (sizeof...(tail) ? "," : ""); debug_impl(tail...); }
	#define debug(...) do { std::cerr << "(" << #__VA_ARGS__ << ") ="; debug_impl(__VA_ARGS__); } while (false)
#else
	#define debug(...) do {} while (false)
#endif
//#define int long long
using ll = long long;
//constexpr int INF = 1e9;//INT_MAX=(1<<31)-1=2147483647
constexpr ll INF = (ll)1e18;//(1LL<<63)-1=9223372036854775807
constexpr ll MOD = (ll)1e9 + 7;
constexpr double EPS = 1e-9;
constexpr ll dx[4] = {1, 0, -1, 0};
constexpr ll dy[4] = {0, 1, 0, -1};
constexpr ll dx8[8] = {1, 0, -1, 0, 1, 1, -1, -1};
constexpr ll dy8[8] = {0, 1, 0, -1, 1, -1, 1, -1};
#define rep(i, n)   for(ll i=0, i##_length=(n); i< i##_length; ++i)
#define repeq(i, n) for(ll i=1, i##_length=(n); i<=i##_length; ++i)
#define rrep(i, n)   for(ll i=(n)-1; i>=0; --i)
#define rrepeq(i, n) for(ll i=(n)  ; i>=1; --i)
#define all(v) (v).begin(), (v).end()
#define rall(v) (v).rbegin(), (v).rend()
void p() { std::cout << '\n'; }
template<typename Head, typename... Tail> void p(Head head, Tail... tail) { std::cout << head << (sizeof...(tail) ? " " : ""); p(tail...); }
template<typename T> inline void pv(std::vector<T>& v) { for(ll i=0, N=v.size(); i<N; i++) std::cout << v[i] << " \n"[i==N-1]; }
template<typename T> inline bool chmax(T& a, T b) { return a < b && (a = b, true); }
template<typename T> inline bool chmin(T& a, T b) { return a > b && (a = b, true); }
template<typename T> inline void uniq(std::vector<T>& v) { v.erase(std::unique(v.begin(), v.end()), v.end()); }

/*-----8<-----template-----8<-----*/

class Edge {
public:
	ll from,to,cost;
	Edge() {}
	Edge(ll from, ll to, ll cost) : from(from), to(to), cost(cost) {}
};
ostream& operator<<(ostream& s, const Edge& e) {
	s << "{ " << e.from << " -> " << e.to << ", " << e.cost << " }";
	return s;
}

/*
計算量：O(ElogV)
引数
　g:探索するグラフ
　start:探索するスタートノード番号
戻り値
　dist:スタートノードから各頂点までの距離
　prev:最短路木の親頂点
*/
void bfs01(const vector<vector<Edge>> &g, ll start, vector<ll> &dist, vector<ll> &prev) {
	ll gsize = g.size();
	dist.assign(gsize, INF); dist[start] = 0;
	prev.assign(gsize, -1);

	deque<ll> que;
	que.emplace_back(start);
	while (!que.empty()) {
		ll e = que.front();
		que.pop_front();
		for(auto&& f : g[e]) {
			if (dist[f.to] > dist[e] + f.cost) {
				dist[f.to] = dist[e] + f.cost;
				prev[f.to] = e;
				if(f.cost) que.emplace_back(f.to);
				else que.emplace_front(f.to);
			}
		}
	}
}
//最短路木の親頂点を元にstart->goalの経路を作成
vector<ll> buildPath(const vector<ll> &prev, ll goal) {
	vector<ll> path;
	for (ll u = goal; u >= 0; u = prev[u])
		path.push_back(u);
	reverse(path.begin(), path.end());
	return path;
}

////


template< typename Graph >
class LowestCommonAncestor {
public:
	ll n;
	std::vector<ll> path, in_order;
	std::vector<std::pair<ll,ll>> depth;
	std::vector< std::vector< std::pair<ll,ll> > > st;
	std::vector< ll > lookup;
	LowestCommonAncestor(const Graph &g, ll root) : n(g.size()), path(n * 2 - 1), in_order(n), depth(n * 2 - 1) {
		ll k = 0;
		dfs(g, root, -1, 0, k);
		build(depth);
	}

	ll query(ll u, ll v) const {
		ll l = std::min(in_order[u], in_order[v]);
		ll r = std::max(in_order[u], in_order[v]) + 1;
		std::pair<ll,ll> t = rmq(l, r);
		return path[t.second];
	}

	void dfs(const Graph &g, ll v, ll p, ll d, ll &k) {
		in_order[v] = k;
		path[k] = v;
		depth[k] = {d, k};
		++k;
		for (auto &nextv : g[v]) {
			if (nextv != p) {
				dfs(g, nextv, v, d + 1, k);
				path[k] = v;
				depth[k] = {d, k};
				++k;
			}
		}
	}

	void build(const vector< std::pair<ll,ll> > &v) {
		ll b = 0;
		while((1 << b) <= (ll)v.size()) ++b;
		st.assign(b, std::vector< std::pair<ll,ll> >(1 << b));
		for(ll i = 0; i < (ll)v.size(); i++) {
			st[0][i] = v[i];
		}
		for(ll i = 1; i < b; i++) {
			for(ll j = 0; j + (1 << i) <= (1 << b); j++) {
				st[i][j] = std::min(st[i - 1][j], st[i - 1][j + (1 << (i - 1))]);
			}
		}
		lookup.resize(v.size() + 1);
		for(ll i = 2; i < (ll)lookup.size(); i++) {
			lookup[i] = lookup[i >> 1] + 1;
		}
	}

	std::pair<ll,ll> rmq(ll l, ll r) const {
		ll b = lookup[r - l];
		return std::min(st[b][l], st[b][r - (1 << b)]);
	}
};


/*-----8<-----library-----8<-----*/
 
const ll N_MAX = 1e5;
 
ll N, startT, startA;
vector<ll> A;
vector<ll> B;
 
void solve() {
	scanf("%lld%lld%lld", &N, &startT, &startA);
	startT--;startA--;
	A.assign(N, 0);
	B.assign(N, 0);
	rep(i, N-1) { scanf("%lld%lld", &A[i], &B[i]); A[i]--;B[i]--;}

	vector<vector<Edge>> g(N);
	vector<vector<ll>> glca(N);
	rep(i,N-1){
		Edge e(A[i],B[i],1);
		Edge e1(B[i],A[i],1);
		g[A[i]].push_back(e);
		g[B[i]].push_back(e1);
		glca[A[i]].push_back(B[i]);
		glca[B[i]].push_back(A[i]);
	}

	vector<ll> dist, prev;
	bfs01(g, startT, dist, prev);

	vector<ll> distA, prevA;
	bfs01(g, startA, distA, prevA);


	LowestCommonAncestor< vector<vector<ll>> > lca(glca,0);

	vector<ll> dis(N,-1);
	rep(i,N){
		ll lcav=lca.query(startT,i);
		if(distA[lcav] > dist[lcav]){
			dis[i]=distA[i];
		}
	}
	ll maxval=-1;
	rep(i,N){
		chmax(maxval,dis[i]);
	}
	ll ans = maxval-1;
	printf("%lld\n", ans);
}
 
// https://atcoder.jp/contests/abc148/tasks/abc148_f
signed main() {
	solve();
	return 0;
}


Measured memory_bytes: 114188