#include<bits/stdc++.h>
using namespace std;using ll=long long;using vi=vector<int>;using vvi=vector<vi>;using vl=vector<ll>;using vvl=vector<vl>;using P=pair<int,int>;using PL=pair<ll,ll>;using vp=vector<P>;using vpl=vector<PL>;
#define pqueue priority_queue
template<typename T>constexpr auto inf=numeric_limits<T>::max()/2;constexpr int INF=inf<int>,MOD=1000000007;constexpr ll LINF=inf<ll>;
#define _ol3(_1,_2,_3,name,...)name
#define _rep(i,n)_repi(i,0,n)
#define _repi(i,a,b)for(int i=a,i##_l=(b);i<i##_l;++i)
#define REP(...)_ol3(__VA_ARGS__,_repi,_rep,)(__VA_ARGS__)
#define REPR(i,n)for(int i=n-1;i>=0;--i)
#define REPA(i,v)REP(i,(v).size())
#define all(v)(v).begin(),(v).end()
#define rall(v)(v).rbegin(),(v).rend()
#define bit(n)(1ll<<(n))
#define F first
#define S second
#define endl '\n'
#define cho(n,a,b)cout<<((n)?a:b)<<endl
void YES(int n){cho(n,"YES","NO");}void Yes(int n){cho(n,"Yes","No");}void Poss(int n){cho(n,"Possible","Impossible");}
void _print(ostream&){}template<class T,class...U>void _print(ostream&s,const T&t,const U&...u){s<<t<<(sizeof...(u)?' ':'\n');_print(s,u...);}
template<class...T>void print(const T&...t){_print(cout,t...);}template<class...T>void dprint(const T&...t){_print(cerr,t...);}
#ifndef LOCAL
struct osd{template<class T>osd&operator<<(const T&t){return*this;}};osd cer_;
#define dprint(...)
#define cerr cer_
#endif
#define dbg(x) dprint("@l",__LINE__,':',#x,'=',x)
template<class T>bool chmax(T&a,const T&b){bool x=a<b;x?a=b:b;return x;}template<class T>bool chmin(T&a,const T&b){bool x=a>b;x?a=b:b;return x;}
template<class T,class U>ostream&operator<<(ostream&o,const pair<T,U>&p){return o<<p.F<<' '<<p.S;}
template<class T,class U>istream&operator>>(istream&i,pair<T,U>&p){return i>>p.F>>p.S;}
template<class T>class iterable{static false_type c(string v);template<class U>static auto c(U v)->decltype(all(v),true_type());static false_type c(...);public:const static bool value=decltype(c(declval<T>()))::value;};
template<class T,enable_if_t<iterable<T>::value,int> =0>ostream&operator<<(ostream&o,const T&v){for(auto&&i:v)o<<i<<' ';return o;}
template<class T>istream&operator>>(istream&i,vector<T>&v){for(T&j:v)i>>j;return i;}template<class T>vector<T>&operator<<(vector<T>&v,const T&t){v.push_back(t);return v;}
template<class T>queue<T>&operator<<(queue<T>&v, const T&t){v.push(t);return v;}template<class T>queue<T>&operator>>(queue<T>&v, T&t){t=v.front();v.pop();return v;}
template<class T>pqueue<T>&operator<<(pqueue<T>&v, const T&t){v.push(t);return v;}template<class T>pqueue<T>&operator>>(pqueue<T>&v, T&t){t=v.front();v.pop();return v;}
template<class T>stack<T>&operator<<(stack<T>&v, const T&t){v.push(t);return v;}template<class T>stack<T>&operator>>(stack<T>&v, T&t){t=v.front();v.pop();return v;}
template<class T>set<T>&operator<<(set<T>&v,const T&t){v.insert(t);return v;}template<class T>multiset<T>&operator<<(multiset<T>&v,const T&t){v.insert(t);return v;}
template<class T,enable_if_t<is_integral<T>::value,int> =0>T mod(T a, T b){if(a>0)return a%b;return b-(-a%b);}
template<class T,enable_if_t<is_floating_point<T>::value,int> =0>T mod(T a, T b){if(a>0)return a%b;return b-fmod(-a,b);}
template<class T,class U>pair<T,U>&operator+=(pair<T,U>&a, pair<T,U>&b){a.F+=b.F;a.S+=b.S;return a;}template<class T,class U>pair<T,U>operator+(const pair<T,U>&a, const pair<T,U>&b){T r=a;return r+=b;}
template<class T,class U>pair<T,U>&operator-=(pair<T,U>&a, pair<T,U>&b){a.F-=b.F;a.S-=b.S;return a;}template<class T,class U>pair<T,U>operator-(const pair<T,U>&a, const pair<T,U>&b){T r=a;return r-=b;}
template<class T,class U>pair<T,U>operator-(pair<T,U>&a){return mp(-a.F, -a.S);}

using len_type=ll;

vvi graph;
void dijkstra(int start, vector<len_type>& dist){
    int n=graph.size();
    fill(all(dist), inf<len_type>);
    dist.resize(n, inf<len_type>);
    priority_queue<P,vector<P>,greater<P>> q;
    dist[start]=0;
    q.push(make_pair(0,start));
    while(!q.empty()){
        P p=q.top();
        q.pop();
        int v=p.second;
        if(dist[v]<p.first)continue;
        for(const int&e:graph[v]){
            if(dist[e]>dist[v]+1){
                dist[e]=dist[v]+1;
                q.push(make_pair(dist[e],e));
            }
        }
    }
}
class UnionFind{
private:
    vi data;
public:
    UnionFind(int size):data(size,-1){}
    bool unite(int x,int y){
        x=root(x);y=root(y);
        if(x!=y){
            if(data[y]<data[x])swap(x,y);
            data[x]+=data[y];data[y]=x;
        }
        return x!=y;
    }
    bool find(int x,int y){
        return root(x)==root(y);
    }
    int root(int x){
        return data[x]<0?x:data[x]=root(data[x]);
    }
    int size(int x){
        return -data[root(x)];
    }
};
int main(){
    cin.tie(0);ios::sync_with_stdio(0);
    cout<<setprecision(10);
    int n,u,v;
    cin>>n>>u>>v;
    --u;--v;
    graph.resize(n);
    REP(i,n-1){
        int a,b;
        cin>>a>>b;
        --a;--b;
        graph[a]<<b;
        graph[b]<<a;
    }
    vl du,dv;
    dijkstra(u,du);
    dijkstra(v,dv);
    // bool f=(dv[u]%2);



    // REP(i,n)x[i]=(du[i]<dv[i]);
    vi d(n,-1);
    d[u]=0;
    queue<int> q;
    q<<u;
    while(q.size()){
        int ds;
        q>>ds;
        for(int e:graph[ds]){
            if(d[e]==-1){
                if(du[e]<=dv[e]){
                    d[e]=d[ds]+1;
                    if(du[e]<dv[e])
                        q<<e;
                }
            }
        }
    }
    //dbg(d);
    ll ma=0;
    REP(i,n)if(d[i]>=0)chmax(ma, dv[i]);
    print(ma-1);
    //cout<<*max_element(all(d));


}


Measured memory_bytes: 7976