#include <bits/stdc++.h>
#define LLI long long int
#define FOR(v, a, b) for(LLI v = (a); v < (b); ++v)
#define FORE(v, a, b) for(LLI v = (a); v <= (b); ++v)
#define REP(v, n) FOR(v, 0, n)
#define REPE(v, n) FORE(v, 0, n)
#define REV(v, a, b) for(LLI v = (a); v >= (b); --v)
#define ALL(x) (x).begin(), (x).end()
#define RALL(x) (x).rbegin(), (x).rend()
#define ITR(it, c) for(auto it = (c).begin(); it != (c).end(); ++it)
#define RITR(it, c) for(auto it = (c).rbegin(); it != (c).rend(); ++it)
#define EXIST(c,x) ((c).find(x) != (c).end())
#define fst first
#define snd second
#define popcount __builtin_popcount
#define UNIQ(v) (v).erase(unique(ALL(v)), (v).end())
#define bit(i) (1LL<<(i))

#ifdef DEBUG
#include <misc/C++/Debug.cpp>
#else
#define dump(...) ((void)0)
#endif

#define gcd __gcd

using namespace std;
template <class T> constexpr T lcm(T m, T n){return m/gcd(m,n)*n;}

template <typename I> void join(ostream &ost, I s, I t, string d=" "){for(auto i=s; i!=t; ++i){if(i!=s)ost<<d; ost<<*i;}ost<<endl;}
template <typename T> istream& operator>>(istream &is, vector<T> &v){for(auto &a : v) is >> a; return is;}
template <typename T, typename U> bool chmin(T &a, const U &b){return (a>b ? a=b, true : false);}
template <typename T, typename U> bool chmax(T &a, const U &b){return (a<b ? a=b, true : false);}
template <typename T, size_t N, typename U> void fill_array(T (&a)[N], const U &v){fill((U*)a, (U*)(a+N), v);}

struct Init{
  Init(){
    cin.tie(0);
    ios::sync_with_stdio(false);
    cout << fixed << setprecision(12);
    cerr << fixed << setprecision(12);
  }
}init;


template <typename Cost = int> class Edge{
public:
  int from,to;
  Cost cost;
  Edge() {}
  Edge(int to, Cost cost): to(to), cost(cost){}
  Edge(int from, int to, Cost cost): from(from), to(to), cost(cost){}

  Edge rev() const {return Edge(to,from,cost);}
  
  friend std::ostream& operator<<(std::ostream &os, const Edge &e){
    os << "(FROM: " << e.from << "," << "TO: " << e.to << "," << "COST: " << e.cost << ")";
    return os;
  }
};

template <typename T> using Graph = std::vector<std::vector<Edge<T>>>;
template <typename T> using Tree = std::vector<std::vector<Edge<T>>>;

template <typename C, typename T> void add_edge(C &g, int from, int to, T w){
  g[from].push_back(Edge<T>(from, to, w));  
}

template <typename C, typename T> void add_undirected(C &g, int a, int b, T w){
  g[a].push_back(Edge<T>(a, b, w));
  g[b].push_back(Edge<T>(b, a, w));
}


template <typename T> class LCA{
private:
  std::vector<std::vector<int>> parent;
  int n, log2n;
  
  void dfs(const Tree<T> &tree, int cur, int par, int d){
    parent[cur][0] = par;
    depth[cur] = d;

    for(auto &e : tree[cur]){
      if(e.to != par){
        dist[e.to] = dist[cur] + e.cost;
        dfs(tree, e.to, cur, d+1);
      }
    }
  }
  
public:
  std::vector<int> depth;
  std::vector<T> dist;
  
  LCA(const Tree<T> &tree, int root):
    n(tree.size()), depth(n), dist(n)
  {
    log2n = (int)ceil(log(n) / log(2)) + 1;
    parent = std::vector<std::vector<int>>(n, std::vector<int>(log2n, 0));

    dfs(tree, root, -1, 0);
    for(int k = 0; k < log2n-1; ++k){
      for(int v = 0; v < n; ++v){
        if(parent[v][k] == -1) parent[v][k+1] = -1;
        else parent[v][k+1] = parent[parent[v][k]][k];
      }
    }
  }

  int query(int a, int b){
    if(depth[a] >= depth[b]) std::swap(a,b);
    for(int k = 0; k < log2n; ++k) if((depth[b] - depth[a]) >> k & 1) b = parent[b][k];
    if(a == b) return a;
    for(int k = log2n-1; k >= 0; --k) if(parent[a][k] != parent[b][k]){a = parent[a][k]; b = parent[b][k];}
    return parent[a][0];
  }

  T distance(int a, int b){
    return dist[a] + dist[b] - 2 * dist[query(a,b)];
  }
};


template <typename T>
void rooting(Tree<T> &tree, int cur, int par = -1){
  if(par != -1){
    for(auto it = tree[cur].begin(); it != tree[cur].end(); ++it){
      if(it->to == par){
        tree[cur].erase(it);
        break;
      }
    }
  }

  for(auto &e : tree[cur]){
    rooting(tree, e.to, cur);
  }
}


template <typename F>
struct FixPoint : F{
  explicit constexpr FixPoint(F &&f) noexcept : F(std::forward<F>(f)){}

  template <typename... Args>
  constexpr decltype(auto) operator()(Args &&... args) const {
    return F::operator()(*this, std::forward<Args>(args)...);
  }
};

template <typename F>
static inline constexpr decltype(auto) make_fix_point(F &&f){
  return FixPoint<F>(std::forward<F>(f));
}


int main(){
  int N, u, v;
  while(cin >> N >> u >> v){
    --u, --v;
    Tree<int> tree(N);
    REP(i,N-1){
      int a,b; cin >> a >> b;
      --a, --b;
      add_undirected(tree, a, b, 1);
    }

    /*
    vector<int> path;
    auto dfs =
      make_fix_point([&](auto &&dfs, int cur, int par) -> bool{
                       if(cur == u){
                         path.push_back(cur);
                         return true;
                       }
                       
                       for(auto &e : tree[cur]){
                         if(e.to == par) continue;

                         if(dfs(e.to, cur)){
                           path.push_back(cur);
                           return true;
                         }
                       }
                       return false;
                                                                
                     });
    dfs(v, -1);

    reverse(ALL(path));
    dump(path);


    auto depth =
      make_fix_point([&](auto &&dfs, int cur, int par) -> int{
                       int ret = 0;

                       for(auto &e : tree[cur]){
                         if(e.to == par) continue;

                         chmax(ret, dfs(e.to, cur));
                       }
                       ret += 1;

                       return ret;
                     });


    int ans = 0;

    FOR(i,1,path.size()-1){
      if(i > (int)path.size()-i-1){
        int cur = path[i];
        dump(cur);
        for(auto &e : tree[cur]){
          if(e.to != path[i-1] and e.to != path[i+1]){
            int d = depth(e.to, cur);
            dump(d);

            if(i % 2 == ((int)path.size()-i-1) % 2){
              chmax(ans, i+d);
            }else{

              chmax(ans, i+d-1);
            }
              
          }
        }
      }
    }


    {
      int d = depth(u, path[path.size()-2]) - 1;
      dump(d);

      if(path.size() % 2 == 0){
        d + path.size() - 1
      }
 
      

      
    }



    

    cout << ans << endl;*/
    

    auto lca = LCA<int>(tree, 0);

    int ans = 0;

    int t = INT_MAX;

    rooting(tree, v);

    REP(i,N){
      if(lca.distance(v,i) > lca.distance(u,i)){
        if(tree[i].size()) chmax(ans, lca.distance(v, i));
      }
      if(lca.distance(v,i) == lca.distance(u,i)){
        chmin(t, lca.distance(v,i));
      }
    }

    if(t != INT_MAX) chmax(ans, t);

    cout << ans << endl;

    /*
    int p = lca.query(u, v);

    if(lca.distance(u, p) >= lca.distance(v, p)){
      int ans = lca.distance(u, v);


      vector<int> d(N);

      auto dfs =
        make_fix_point([&](auto &&dfs, int cur) -> void{
                         d[cur] = 0;

                         for(auto &e : tree[cur]){
                           dfs(e.to);
                           chmax(d[cur], d[e.to]);
                         }

                         d[cur] += 1;
                       });
      dfs(u);
      
      ans += *max_element(ALL(d));
      
      

      cout << ans << endl;
    }else{

      cout << -1 << endl;
    }
    */
    

    
  }

  return 0;
}


Measured memory_bytes: 20864