#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);
  }
}


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);
    }

    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;
  }

  return 0;
}


Measured memory_bytes: 20864