#include<bits/stdc++.h>
using namespace std;
typedef long long ll;
typedef long double ld;
typedef pair<ll,ll> P;
typedef pair<int,int> Pi;
#define rep(i,n) for(ll i=0;i<n;i++)
#define FOR(i,a,b) for(ll i=a;i<b;i++)
#define fi first
#define se second

template<typename T> inline bool chmax(T &a, T b){if(a<b){a=b;return true;}return false;}
template<typename T> inline bool chmin(T &a, T b){if(a>b){a=b;return true;}return false;}
template<typename T> ostream& operator<<(ostream& s,const complex<T>& d) {return s<<"("<<d.real()<<", "<<d.imag()<< ")";}
template<typename T1, typename T2> ostream& operator<<(ostream& s,const pair<T1,T2>& d) {return s<<"("<<d.first<<", "<<d.second<<")";}
template<typename T> ostream& operator<<(ostream& s, const vector<T>& d){int len=d.size();rep(i,len){s<<d[i];if(i<len-1) s<<" ";}return s;}
template<typename T> ostream& operator<<(ostream& s,const vector<vector<T>>& d){int len=d.size();rep(i,len){s<<d[i]<<endl;}return s;}
template<typename T> ostream& operator<<(ostream& s,const set<T>& v){s<<"{ ";for(auto itr=v.begin();itr!=v.end();++itr) {if (itr!=v.begin()) {s<< ", ";}s<<(*itr);}s<<" }";return s;}
template<typename T1, typename T2> ostream& operator<<(ostream& s,const map<T1,T2>& m){s<<"{"<<endl;for(auto itr=m.begin();itr!=m.end();++itr){s<<" "<<(*itr).first<<" : "<<(*itr).second<<endl;}s<<"}"<<endl;return s;}

const ll mod=1e9+7;
const ll inf=1e17;
const int INF=1e9;
const double PI=acos(-1);
const double EPS=1e-10;

int main(){
	cin.tie(0);ios::sync_with_stdio(false);
	int n,u,v;
	cin>>n>>u>>v;
	u--,v--;
	vector<vector<int>> g(n);
	rep(i,n-1){
		int a,b;
		cin>>a>>b;
		a--,b--;
		g[a].push_back(b);
		g[b].push_back(a);
	}
	if(u==v){
		cout<<0<<endl;
		return 0;
	}
	vector<vector<int>> dist(2,vector<int>(n,INF));
	auto dfs=[&](auto &dfs,int v,int pre,int flag,int now)->void{
		dist[flag][v]=now;
		for(auto itr:g[v]){
			if(itr==pre) continue;
			dfs(dfs,itr,v,flag,now+1);
		}
	};
	dfs(dfs,u,-1,0,0);
	dfs(dfs,v,-1,1,0);
	int d=-1;
	int ans=-1;
	rep(i,n){
		if(g[i].size()==1){
			if(dist[0][i]<dist[1][i]){
				if(chmax(d,dist[0][i])){
					ans=i;
				}
			}
		}
	}
	cout<<dist[1][ans]-1<<endl;
}