#include<iostream>
#include<string>
#include<vector>
#include<algorithm>
#include<map>
#include<set>
#include<cstdio>
#include<cmath>
#include<numeric>
#include<queue>
#include<stack>
#include<cstring>
#include<limits>
#include<functional>
#include<unordered_set>
#include<iomanip>
#include<cassert>
#include<regex>
#include<bitset>
#include<chrono>
#define rep(i,a) for(int i=(int)0;i<(int)a;++i)
#define pb push_back
#define eb emplace_back
using ll=long long;
constexpr ll mod = 1e9 + 7;
constexpr ll INF = 1LL << 50;
 
template<class T> inline bool chmin(T& a, T b) {
    if (a > b) {
        a = b;
        return true;
    }
    return false;
}
template<class T> inline bool chmax(T& a, T b) {
    if (a < b) {
        a = b;
        return true;
    }
    return false;
}
 
using namespace std;
 
template <typename A, typename B>
string to_string(pair<A, B> p);
 
template <typename A, typename B, typename C>
string to_string(tuple<A, B, C> p);
 
template <typename A, typename B, typename C, typename D>
string to_string(tuple<A, B, C, D> p);
 
string to_string(const string& s) {
  return '"' + s + '"';
}
 
string to_string(const char* s) {
  return to_string((string) s);
}
 
string to_string(bool b) {
  return (b ? "true" : "false");
}
 
string to_string(vector<bool> v) {
  bool first = true;
  string res = "{";
  for (int i = 0; i < static_cast<int>(v.size()); i++) {
    if (!first) {
      res += ", ";
    }
    first = false;
    res += to_string(v[i]);
  }
  res += "}";
  return res;
}
 
template <size_t N>
string to_string(bitset<N> v) {
  string res = "";
  for (size_t i = 0; i < N; i++) {
    res += static_cast<char>('0' + v[i]);
  }
  return res;
}
 
template <typename A>
string to_string(A v) {
  bool first = true;
  string res = "{";
  for (const auto &x : v) {
    if (!first) {
      res += ", ";
    }
    first = false;
    res += to_string(x);
  }
  res += "}";
  return res;
}
 
template <typename A, typename B>
string to_string(pair<A, B> p) {
  return "(" + to_string(p.first) + ", " + to_string(p.second) + ")";
}
 
template <typename A, typename B, typename C>
string to_string(tuple<A, B, C> p) {
  return "(" + to_string(get<0>(p)) + ", " + to_string(get<1>(p)) + ", " + to_string(get<2>(p)) + ")";
}
 
template <typename A, typename B, typename C, typename D>
string to_string(tuple<A, B, C, D> p) {
  return "(" + to_string(get<0>(p)) + ", " + to_string(get<1>(p)) + ", " + to_string(get<2>(p)) + ", " + to_string(get<3>(p)) + ")";
}
 
void debug_out() { cerr << endl; }
 
template <typename Head, typename... Tail>
void debug_out(Head H, Tail... T) {
  cerr << " " <<to_string(H);
  debug_out(T...);
}
#ifdef LOCAL
#define debug(...) cerr << "[" << #__VA_ARGS__ << "]:", debug_out(__VA_ARGS__)
#else
#define debug(...) 42
#endif

template< typename T >
struct edge {
  int src, to;
  T cost;

  edge(int to, T cost) : src(-1), to(to), cost(cost) {}

  edge(int src, int to, T cost) : src(src), to(to), cost(cost) {}

  edge &operator=(const int &x) {
    to = x;
    return *this;
  }

  operator int() const { return to; }
};

template< typename T >
using Edges = vector< edge< T > >;
template< typename T >
using WeightedGraph = vector< Edges< T > >;
using UnWeightedGraph = vector< vector< int > >;
template< typename T >
using Matrix = vector< vector< T > >;


vector<int>pre;

template<typename T>
vector<T> dijkstra(vector<vector<edge<T>>> &g,int s){//sは始点
  const auto INFTY=numeric_limits<T>::max();
  vector<T>dist(g.size(),INFTY);
  pre.resize(g.size(),-1);
  using P=pair<T,int>;//firstは最短距離,secondは頂点の番号
  priority_queue<P,vector<P>,greater<P>>pq;
  dist[s]=0;
  pq.emplace(dist[s],s);

  while(!pq.empty()){
    auto x=pq.top();pq.pop();
    int v=x.second;
    if(dist[v]<x.first)continue;//最短距離でなければ飛ばす
    for(auto e:g[v]){
      if(dist[e.to]>dist[v]+e.cost){
        pre[e.to]=v;
        dist[e.to]=dist[v]+e.cost;
        pq.emplace(dist[e.to],e.to);
      }
    }
  }
  return dist;
}

template<typename T>
vector<T>get_path(int t){//頂点tへの最短経路
    vector<T>path;
    for(;t!=-1;t=pre[t]){
      path.push_back(t);
    }
    reverse(path.begin(),path.end());
    return path;
}

void solve(){
    int n,u,v;
    cin>>n>>u>>v;
    u--;v--;
    WeightedGraph<int>g(n);
    int a,b;
    rep(i,n-1){
      cin>>a>>b;
      a--;b--;
      g[a].eb(b,1);
      g[b].eb(a,1);
    }
    int ans=0;
    vector<int>s=dijkstra(g,v);
    vector<int>t=dijkstra(g,u);
    rep(i,n){
      if(t[i]<=s[i])chmax(ans,s[i]);
    }
    cout<<ans-1<<"\n";
}
 
int main(){
	ios::sync_with_stdio(false);
    cin.tie(0);
	cout<<fixed<<setprecision(15);
	solve();
	return 0;
}

Measured memory_bytes: 8240