#include <bits/stdc++.h>

using namespace std;

using int64 = long long;
const int mod = 104857601;


const int64 infll = (1LL << 62) - 1;
const int inf = (1 << 30) - 1;

struct IoSetup {
  IoSetup() {
    cin.tie(nullptr);
    ios::sync_with_stdio(false);
    cout << fixed << setprecision(10);
    cerr << fixed << setprecision(10);
  }
} iosetup;


template< typename T1, typename T2 >
ostream &operator<<(ostream &os, const pair< T1, T2 > &p) {
  os << p.first << " " << p.second;
  return os;
}

template< typename T1, typename T2 >
istream &operator>>(istream &is, pair< T1, T2 > &p) {
  is >> p.first >> p.second;
  return is;
}

template< typename T >
ostream &operator<<(ostream &os, const vector< T > &v) {
  for(int i = 0; i < (int) v.size(); i++) {
    os << v[i] << (i + 1 != v.size() ? " " : "");
  }
  return os;
}

template< typename T >
istream &operator>>(istream &is, vector< T > &v) {
  for(T &in : v) is >> in;
  return is;
}

template< typename T1, typename T2 >
inline bool chmax(T1 &a, T2 b) { return a < b && (a = b, true); }

template< typename T1, typename T2 >
inline bool chmin(T1 &a, T2 b) { return a > b && (a = b, true); }

template< typename T = int64 >
vector< T > make_v(size_t a) {
  return vector< T >(a);
}

template< typename T, typename... Ts >
auto make_v(size_t a, Ts... ts) {
  return vector< decltype(make_v< T >(ts...)) >(a, make_v< T >(ts...));
}

template< typename T, typename V >
typename enable_if< is_class< T >::value == 0 >::type fill_v(T &t, const V &v) {
  t = v;
}

template< typename T, typename V >
typename enable_if< is_class< T >::value != 0 >::type fill_v(T &t, const V &v) {
  for(auto &e : t) fill_v(e, v);
}

template< typename F >
struct FixPoint : F {
  FixPoint(F &&f) : F(forward< F >(f)) {}

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

template< typename F >
inline decltype(auto) MFP(F &&f) {
  return FixPoint< F >{forward< F >(f)};
}

template< typename key_t, typename val_t >
struct RadixHeap {
  static constexpr int bit = sizeof(key_t) * 8;
  array< vector< pair< key_t, val_t > >, bit > vs;

  size_t sz;
  key_t last;

  RadixHeap() : sz(0), last(0) {}

  bool empty() const { return sz == 0; }

  size_t size() const { return sz; }

  inline int getbit(int a) const {
    return a ? bit - __builtin_clz(a) : 0;
  }

  inline int getbit(int64_t a) const {
    cout << "foo" << endl;
    return a ? bit - __builtin_clzll(a) : 0;
  }

  void push(const key_t &key, const val_t &val) {
    sz++;
    vs[getbit(key ^ last)].emplace_back(key, val);
  }

  pair< key_t, val_t > pop() {
    if(vs[0].empty()) {
      int idx = 1;
      while(vs[idx].empty()) idx++;
      last = min_element(vs[idx].begin(), vs[idx].end())->first;
      for(auto &p:vs[idx]) vs[getbit(p.first ^ last)].emplace_back(p);
      vs[idx].clear();
    }
    --sz;
    auto res = vs[0].back();
    vs[0].pop_back();
    return res;
  }
};

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 > >;

template< typename T >
vector< T > dijkstra_radix_heap(WeightedGraph< T > &g, int s) {
  const auto INF = numeric_limits< T >::max();
  vector< T > dist(g.size(), INF);

  using Pi = pair< T, int >;
  RadixHeap< T, int > heap;
  dist[s] = 0;
  heap.push(dist[s], s);
  while(!heap.empty()) {
    T cost;
    int idx;
    tie(cost, idx) = heap.pop();
    if(dist[idx] < cost) continue;
    for(auto &e : g[idx]) {
      auto next_cost = cost + e.cost;
      if(dist[e.to] <= next_cost) continue;
      dist[e.to] = next_cost;
      heap.push(dist[e.to], e.to);
    }
  }
  return dist;
}


int main() {
  
    int n, t, a; cin>>n>>t>>a;
    t--; a--;
    WeightedGraph< int > g(n);
    for(int i = 0; i < n-1; i++) {
      int x,y; cin>>x>>y; x--; y--;
      g[x].emplace_back(y, 1);
      g[y].emplace_back(x, 1);
    }
    auto cost = dijkstra_radix_heap(g, t);
    auto cost2 = dijkstra_radix_heap(g, a);
    int l = -1 , r = 114514;
    while(abs(l - r) > 1){
        int mid = (r + l) / 2;
        
        bool ok = false; //高橋くんは逃げられるか
        for(int i = 0;i<n;i++){
            if(cost[i]%2 != mid%2 || cost[i] > mid) continue;
            if(cost2[i] > mid && g[i].size()!= 1){
                ok = true;
            }
            if(cost2[i] > mid + 1){
                ok = true;
            }
        }

        if(ok) l = mid;
        else r = mid;
    }
    cout<<r<<endl;
}

Measured memory_bytes: 8420