#include <iostream>
#include <iomanip>
#include <string>
#include <stack>
#include <vector>
#include <math.h>
#include <stdio.h>
#include <algorithm>
#include <utility>
#include <functional>
#include <iterator>
#include <map>
#include <set>
#include <queue>
#include <list>
#include <regex>
using namespace std;
using pii  = pair<int,int>;
using ll=long long;
using ld=long double;
#define pb push_back
#define mp make_pair
#define sc second
#define fr first
#define stpr setprecision
#define cYES cout<<"YES"<<endl
#define cNO cout<<"NO"<<endl
#define cYes cout<<"Yes"<<endl
#define cNo cout<<"No"<<endl
#define rep(i,n) for(ll i=0;i<(n);++i)
#define Rep(i,a,b) for(ll i=(a);i<(b);++i)
#define rrep(i,n) for(int i=n-1;i>=0;i--)
#define rRep(i,a,b) for(int i=a;i>=b;i--)
#define crep(i) for(char i='a';i<='z';++i)
#define psortsecond(A,N) sort(A,A+N,[](const pii &a, const pii &b){return a.second<b.second;});
#define ALL(x) (x).begin(),(x).end()
#define debug(v) cout<<#v<<":";for(auto x:v){cout<<x<<' ';}cout<<endl;
#define endl '\n'
int ctoi(const char c){
  if('0' <= c && c <= '9') return (c-'0');
  return -1;
}
ll gcd(ll a,ll b){return (b == 0 ? a : gcd(b, a%b));}
ll lcm(ll a,ll b){return a*b/gcd(a,b);}
constexpr ll MOD=1000000007;
constexpr ll INF=1000000011;
constexpr ll MOD2=998244353;
constexpr ll LINF = 1001002003004005006ll;
constexpr ld EPS=10e-8;
template<class T>bool chmax(T &a, const T &b) { if (a<b) { a=b; return 1; } return 0; }
template<class T>bool chmin(T &a, const T &b) { if (b<a) { a=b; return 1; } return 0; }
template<typename T> istream& operator>>(istream& is,vector<T>& v){for(auto&& x:v)is >> x;return is;}
template<typename T,typename U> istream& operator>>(istream& is, pair<T,U>& p){ is >> p.first; is >> p.second; return is;}
template<typename T,typename U> ostream& operator>>(ostream& os, const pair<T,U>& p){ os << p.first << ' ' << p.second; return os;}
template<class T> ostream& operator<<(ostream& os, vector<T>& v){
  for(auto i=begin(v); i != end(v); ++i){
    if(i !=begin(v)) os << ' ';
    os << *i;
  }
  return os;
}

struct Edge {
  ll to, cost;
  Edge(ll to, ll cost) : to(to), cost(cost) {}
};

typedef vector<vector<Edge> > AdjList; // 辺の情報
typedef vector<ll> dis_dijkstra; // 距離のvector

dis_dijkstra dijkstra(ll n ,AdjList A,ll st ,ll weigh){// 頂点数 n ,グラフ　A　,始点　st　,初期値 weigh
  dis_dijkstra DIS;
  vector<bool> chek;
  DIS = vector<ll>(n+1, LINF);
  chek = vector<bool>(n+1, 0); 
  DIS[st]=weigh;
  ll key=st;
  priority_queue <pair<ll,ll>> PQ;
  PQ.push(mp(weigh,st));
  while(!PQ.empty()){
    pair<ll,ll> f = PQ.top();
    PQ.pop();
    key=f.second;
    chek[key]=1;
    if(DIS[key] < f.first*(-1)){
      continue;
    }
    rep(j,A[key].size()){
      ll v = A[key][j].to;
      if(chek[v]==0 && DIS[v]>DIS[key]+A[key][j].cost){
        DIS[v] = DIS[key] + A[key][j].cost;
        PQ.push(mp(DIS[v]*(-1),v));
      }
    }
  }
  return DIS;
}

int main(){
    ll N, U , V ;
    cin >> N >> U >> V ;
    AdjList graph;
    graph = AdjList(N+1);
  
    rep(i,N-1){
        ll X, Y;
        cin >> X >> Y ;
        graph[X].push_back(Edge(Y, 1));
        graph[Y].push_back(Edge(X, 1));
    }
    dis_dijkstra dis = dijkstra(N,graph,U,0LL);
    dis_dijkstra dis1 = dijkstra(N,graph,V,0LL);
    ll ma=-1;
    Rep(i,1,N+1){
      if(dis1[i]>dis[i]){
        ma=max(dis1[i],ma);
      }
    }
    cout << ma-1 << endl;
}

Measured memory_bytes: 16800