#include <bits/stdc++.h>
#ifdef LOCAL_DEBUG
#define DEBUG 1
#define CERR if(DEBUG) cerr
#define MACRO(_1,_2,_3,_4,_5,_6,_7,_8,_9,_10,NAME,...) NAME
#define pr(...) CERR<<MACRO(__VA_ARGS__,pr10,pr9,pr8,pr7,pr6,pr5,pr4,pr3,pr2,pr1)(__VA_ARGS__)<<endl
#define pr1(a) (#a)<<"="<<(a)<<" "
#define pr2(a,b) pr1(a)<<pr1(b)
#define pr3(a,b,c) pr1(a)<<pr2(b,c)
#define pr4(a,b,c,d) pr1(a)<<pr3(b,c,d)
#define pr5(a,b,c,d,e) pr1(a)<<pr4(b,c,d,e)
#define pr6(a,b,c,d,e,f) pr1(a)<<pr5(b,c,d,e,f)
#define pr7(a,b,c,d,e,f,g) pr1(a)<<pr6(b,c,d,e,f,g)
#define pr8(a,b,c,d,e,f,g,h) pr1(a)<<pr7(b,c,d,e,f,g,h)
#define pr9(a,b,c,d,e,f,g,h,i) pr1(a)<<pr8(b,c,d,e,f,g,h,i)
#define pr10(a,b,c,d,e,f,g,h,i,j) pr1(a)<<pr9(b,c,d,e,f,g,h,i,j)
#define prArr(a){CERR<<(#a)<<"={";int i=0;for(auto t:(a))CERR<<(i++?", ":"")<<t;CERR<<"}"<<endl;}
#else
#define DEBUG 0
#define pr(...)
#define prArr(a)
#endif
using namespace std;
using Int = long long;
using _int = int;
using ll = long long;
using Double = long double;
const Int INF = (1LL<<60)+1e9; // ~ 1.15 * 1e18
const Int mod = (1e9)+7;
const Double EPS = 1e-8;
const Double PI = 6.0 * asin((Double)0.5);
using P = pair<Int,Int>;
template<class T> T Max(T &a,T b){return a=max(a,b);}
template<class T> T Min(T &a,T b){return a=min(a,b);}
template<class T1, class T2> ostream& operator<<(ostream& o,pair<T1,T2> p){return o<<"("<<p.first<<","<<p.second<<")";}
template<class T1, class T2, class T3> ostream& operator<<(ostream& o,tuple<T1,T2,T3> t){return o<<"("<<get<0>(t)<<","<<get<1>(t)<<","<<get<2>(t)<<")";}
template<class T1, class T2> istream& operator>>(istream& i,pair<T1,T2> &p){return i>>p.first>>p.second;}
template<class T> ostream& operator<<(ostream& o,vector<T> a){Int i=0;for(T t:a)o<<(i++?" ":"")<<t;return o;}
template<class T> istream& operator>>(istream& i,vector<T> &a){for(T &t:a)i>>t;return i;}
//INSERT ABOVE HERE
class LCA{
public:
  typedef pair<int,int> P;
  int LOG_V=20;                       //log V (自分で決めて)
  int V;                              //言わずもがな頂点数
  vector<vector<P> > G;               //グラフの隣接リストの表現
  int root;                           //根のノード番号
  vector<vector<int> > parent;        //親を2^k回辿って到達する頂点(根を通りすぎる場合は-1とする)
  vector<int> depth;                  //根からの深さ
  vector<int> D;                      //根からの距離
  int ok;

  LCA():V(-1){}
  LCA(int V,int root = 0)
    :V(V),G(V),root(root),parent(LOG_V,vector<int>(V)),depth(V), D(V,0),ok(0){}

  void add_edge(int a, int b,int c = 1){
    assert(a >= 0 && b >= 0);
    assert(a < V && b < V);
    G[a].push_back(P(b,c));
    G[b].push_back(P(a,c));
  }

  void dfs(int v,int p,int d,int c){
    assert(v < V);
    parent[0][v] = p;
    depth[v] = d;
    D[v] = c;
    for(auto to:G[v])
      if(to.first != p) dfs(to.first, v, d+1, c + to.second);
  }

  //vからn個上の祖先
  int getAnc(int v,int n){
    for(int k=0; k<LOG_V ;k++) if((v != -1) && (n>>k&1)) v=parent[k][v];
    return v;
  }

  //初期化
  void build(){
    ok = 1;
    //parent[0]とdepthを初期化する。
    dfs(root,-1,0,0);
    //parentを初期化する
    for(int k=0; k+1<LOG_V ;k++)
      for(int v=0; v<V ;v++)
        if(parent[k][v] < 0)parent[k+1][v] = -1;
        else parent[k+1][v] = parent[k][parent[k][v]];
  }

  //uとvのLCAを求める
  int lca(int u,int v){
    assert(ok);
    assert(u >= 0 && v >= 0);
    assert(u < V && v < V);
    if(depth[u]>depth[v]) swap(u,v);
    v = getAnc(v, depth[v] - depth[u]);

    if(u==v) return u;
    //二分探索でLCAを求める
    for(int k=LOG_V-1; k>=0 ;k--)
      if(parent[k][u] != parent[k][v]){
        u = parent[k][u];
        v = parent[k][v];
      }
    return parent[0][u];
  }

  int getDis(int u,int v){
    assert(u >= 0 && v >= 0);
    assert(u < V && v < V);
    return D[u] + D[v] - 2 * D[lca(u, v)];
  }
};

signed main(){
  srand((unsigned)time(NULL));
  cin.tie(0);
  ios_base::sync_with_stdio(0);
  cout << fixed << setprecision(12);

  int N, u, v;
  cin>>N>>u>>v; u--, v--;

  LCA lca(N, v);
  vector<vector<int> > G(N);
  for(int i=0;i<N-1;i++){
    int a, b;
    cin>>a>>b; a--, b--;
    lca.add_edge(a, b);
    G[a].push_back(b);
    G[b].push_back(a);
  }

  lca.build();

  int ans = 0;
  for(int i=0;i<N;i++){
    int m = lca.lca(u, i);
    int disU = lca.getDis(m, u);
    int disV = lca.getDis(m, v);
    if(disU >= disV) continue;
    Max(ans, lca.getDis(v, i) - 1);
  }

  cout<<ans<<endl;


  return 0;
}


Measured memory_bytes: 22648