#pragma GCC optimize("Ofast","unroll-loops")
#pragma comment(linker, "/stack:200000000")
#pragma GCC target("sse,sse2,sse3,ssse3,sse4,popcnt,abm,mmx,avx,tune=native")
#include<bits/stdc++.h>
using namespace::std;
//struct __INIT{__INIT(){cin.tie(0);ios::sync_with_stdio(false);cout<<fixed<<setprecision(15);}} __init;
__attribute__((constructor))
void init(){
	cin.tie(0);
	ios::sync_with_stdio(false);
	cout<<fixed<<setprecision(15);
}
// random_device rd;
// mt19937 mt(rd());
// #include <boost/numeric/interval.hpp>
// #include <boost/numeric/interval/io.hpp>
// #include <boost/intrusive/rbtree.hpp>
// #include <boost/dynamic_bitset.hpp>
// #include <boost/geometry.hpp>
// #include <boost/geometry/geometries/linestring.hpp>
// #include <boost/geometry/geometries/polygon.hpp>
// #include <boost/geometry/geometries/point_xy.hpp>
// #include <boost/math/tools/minima.hpp>
// using boost::math::tools::brent_find_minima;
#include <boost/multiprecision/cpp_dec_float.hpp>
#include <boost/multiprecision/cpp_int.hpp>
namespace mp = boost::multiprecision;
typedef mp::number<mp::cpp_dec_float<0>> cdouble;
typedef mp::cpp_int cint;
// #include <boost/unordered_map.hpp>
#include<ext/pb_ds/assoc_container.hpp>
#include<ext/pb_ds/tree_policy.hpp>
#include <ext/pb_ds/priority_queue.hpp>
#include<ext/pb_ds/tag_and_trait.hpp>
template<typename T=long long>using pbds=__gnu_pbds::tree<T,__gnu_pbds::null_type,less<T>,__gnu_pbds::rb_tree_tag,__gnu_pbds::tree_order_statistics_node_update>;
template<typename T=long long>using pbds_map=__gnu_pbds::tree<T,T,less<T>,__gnu_pbds::rb_tree_tag,__gnu_pbds::tree_order_statistics_node_update>;
template<typename T=long long>using pqueue =__gnu_pbds::priority_queue<T, less<T>,__gnu_pbds::rc_binomial_heap_tag>;
typedef long long lint;
typedef long long ll;
typedef long double ldouble;
typedef vector<lint> vec;
typedef vector<vector<lint>> mat;
typedef vector<vector<vector<lint>>> mat3;
typedef vector<double> dvec;
typedef vector<vector<double>> dmat;
typedef vector<vector<vector<double>>> dmat3;
typedef vector<string> svec;
typedef vector<vector<string>> smat;
typedef vector<vector<vector<string>>> smat3;
template<typename T>using Vec=vector<T>;
template<typename T>using Mat=vector<vector<T>>;
template<typename T>using Mat3=vector<vector<vector<T>>>;
template<typename T>using Mat4=vector<vector<vector<vector<T>>>>;
template<typename S,typename T>using pvec=vector<pair<S,T>>;
template<typename S,typename T>using pmat=vector<vector<pair<S,T>>>;
template<typename S,typename T>using pmat3=vector<vector<vector<pair<S,T>>>>;
template<typename S,typename T>using pmat4=vector<vector<vector<vector<pair<S,T>>>>>;
template<typename... T>using tvec=vector<tuple<T...>>;
template<typename... T>using tmat=vector<vector<tuple<T...>>>;
template<typename... T>using tmat3=vector<vector<vector<tuple<T...>>>>;
template<typename... T>using tmat4=vector<vector<vector<vector<tuple<T...>>>>>;
#define rep(i, n) for(lint i = 0; i < (lint)(n); i++)
#define irep(i) for(lint i = 0;; i++)
#define irep1(i) for(lint i = 1;; i++)
#define irep2(i) for(lint i = 2;; i++)
#define rrep(i, n) for(lint i = (lint)(n-1); i >-1; i--)
#define rrepi(i,a,b) for(lint i = (lint)(b-1); i >a-1; i--)
#define repi(i,a,b) for(lint i=lint(a);i<lint(b);i++)
#define rep2(i,a,b,c) for(lint i=lint(a);i>lint(b);i+=c)
#define all(x) (x).begin(),(x).end()
#define dist(x1,y1,x2,y2) (pow(pow(x2-x1,2)+pow(y2-y1,2),0.5))
//#define input(a,n) lint n;cin>>n;vector<lint>a(n);rep(i,n)cin>>a[i];
#define SUM(v) accumulate(all(v),0LL)
#define INF (1LL<<60)
#define IINF (1<<30)
#define EPS (1e-10)
#define LINF 9223372036854775807LL
//#define MOD 998244353LL
#define MOD 1000000007LL
#define endl "\n"
template<typename T>inline void numout(T t){bool f=0;for(auto i:t){cout<<(f?" ":"")<<i<INF/2?i:"INF";f=1;}cout<<endl;}
template<typename T>inline void numout2(T t){for(auto i:t)numout(i);}
template<typename T>inline void output(T t){bool f=0;for(auto i:t){cout<<(f?" ":"")<<i;f=1;}cout<<endl;}
template<typename T>inline void output2(T t){for(auto i:t)output(i);}
template<typename T>inline void _output(T t){bool f=0;for(int i=0;i<t.size();i++){cout<<f?"":" "<<t[i];f=1;}cout<<endl;}
template<typename T>inline void _output2(T t){for(int i=0;i<t.size();i++)output(t[i]);}
auto call=[](auto f,auto... args){return f(f,args...);};
template<typename T=lint>T in(){return *istream_iterator<T>(cin);}
template<typename T=lint>inline T gcd(T a,T b){return b?gcd(b,a%b):a;}
template<typename T=lint>inline T lcm(T a,T b){return a/gcd(a,b)*b;}
template<typename T>inline T minq(T a,T b){return min(a,b);}
template<typename T>inline T maxq(T a,T b){return max(a,b);}
inline bool chmin(auto& s,const auto& t){bool res=s>t;s=min(s,t);return res;}
inline bool chmax(auto& s,const auto& t){bool res=s<t;s=max(s,t);return res;}
const vector<lint> dx={-1,1,0,0,1,1,-1,-1};
const vector<lint> dy={0,0,-1,1,1,-1,1,-1};

template<typename Int>
class LCA{
    template<typename T>
    class sparse_table{
        using F=function<T(T,T)>;
        F f;
        vector<vector<T>>data;
        public:
        sparse_table(vector<T> v,F f):f(f){
            int n=v.size(),log=log2(n)+1;
            data.resize(n,vector<T>(log));
            for(int i=0;i<n;i++)data[i][0]=v[i];
            for(int j=1;j<log;j++)for(int i=0;i+(1<<(j-1))<n;i++){
                data[i][j]=f(data[i][j-1],data[i+(1<<(j-1))][j-1]);
            }
        }
        T get(int l,int r){
            if(r<l)swap(l,r);
            //assert(0<l||r<=(T)data.size());
            int k=log2(r-l);
            return f(data[l][k],data[r-(1<<k)][k]);
        }
    };
    vector<pair<Int,Int>> data;
    vector<Int> start;
    sparse_table<pair<Int,Int>>* st;
	vector<Int> dep;
	vector<Int> __dist;
    public:
	LCA(){}
    LCA(vector<vector<Int>> v,Int s){
        data.resize(v.size()*2-1);
        start.resize(v.size());
        Int i=0;
        dep=depth(s,v);
		__dist=__distance(s,v);
        auto f=[&](auto f,Int n,Int p)->void{
            start[n]=i;
            data[i++]=make_pair(dep[n],n);
            for(Int t:v[n]){
                if(t==p)continue;
                f(f,t,n);
                data[i++]=make_pair(dep[n],n);
            }
        };
        call(f,s,-1);
        st=new sparse_table<pair<Int,Int>>(data,[](auto s,auto t){return s.first>t.first?s:t;});
    }
    Int lca(auto p,auto q){
        return st->get(min(start[p],start[q]),max(start[p],start[q])+1).second;
    }
	Int distance(auto p,auto q){
        return __dist[p]+__dist[q]-2*__dist[lca(p,q)];
    }
    private:
    vector<Int> __distance(Int start,vector<vector<Int>>G){
		vector<Int>memo(G.size());
		auto f=[&](auto f,Int v,Int p,Int i)->Int{
			for(auto t:G[v]){
				if(t==p)continue;
				f(f,t,v,i+1);
			}
			return memo[v]=i;
		};
		call(f,start,-1,0);
		return memo;
    }
	vector<Int> depth(Int start,vector<vector<Int>>G){
		vector<Int>memo(G.size());
		auto f=[&](auto f,Int v,Int p)->lint{
			Int mx=0;
			for(auto t:G[v]){
				if(t==p)continue;
				mx=max(mx,f(f,t,v));
			}
			return memo[v]=mx+1;
		};
		call(f,start,-1);
		return memo;
	}
};
vector<lint> depth(lint start,vector<vector<lint>>G){
	vector<lint>memo(G.size());
	auto f=[&](auto f,lint v,lint p)->lint{
		lint mx=0;
		for(auto t:G[v]){
			if(t==p)continue;
			mx=max(mx,f(f,t,v));
		}
		return memo[v]=mx+1;
	};
	call(f,start,-1);
	return memo;
}
int main(){
	lint n,u,v;
	cin>>n>>u>>v;
	mat g(n);
	rep(i,n-1){
		lint s,t;
		cin>>s>>t;
		s--;t--;
		g[s].push_back(t);
		g[t].push_back(s);
	}
	u--;v--;
	LCA<lint> lca(g,0);
	lint ans=0;
	auto a=depth(v,g);
	rep(i,5*n){
		if(u==v){
			cout<<i<<endl;
			return 0;
		}
		lint mx=-INF;
		if(g[u].size()==1)u=g[u][0];
		else {
			for(auto d:g[u]){
				if(lca.distance(d,v)<2)continue;
				if(lca.distance(d,v)<lca.distance(u,v)){
					u=d;
					break;
				}else{
					if(chmax(mx,a[d])){
						u=d;
					}
				}
			}
		}
		if(u==v){
			cout<<i<<endl;
			return 0;
		}
		if(g[v].size()==1)v=g[v][0];
		else {
			for(auto d:g[v]){
				if(lca.distance(u,v)==lca.distance(u,d)){
					cout<<u<<v<<d<<endl;
					cout<<lca.distance(u,v)<<endl;
				}
				if(lca.distance(u,v)>lca.distance(u,d))v=d;
			}
		}
	}
	cout<<ans<<endl;
}

Measured memory_bytes: 89944