#include <bits/stdc++.h>
using namespace std;
using ll = long long;
using VI = vector<ll>;
using VV = vector<VI>;
using VS = vector<string>;
using PII = pair<ll, ll>;
template <typename A, typename B>
string to_string(pair<A, B> p);
template <typename A, typename B, typename C>
string to_string(tuple<A, B, C> p);
template <typename A, typename B, typename C, typename D>
string to_string(tuple<A, B, C, D> p);
string to_string(const string &s) { return '"' + s + '"'; }
string to_string(const char *s) { return to_string((string)s); }
string to_string(bool b) { return (b ? "true" : "false"); }
string to_string(vector<bool> v) {
  bool first = true;
  string res = "{";
  for (int i = 0; i < static_cast<int>(v.size()); i++) {
    if (!first) {
      res += ", ";
    }
    first = false;
    res += to_string(v[i]);
  }
  res += "}";
  return res;
}
template <size_t N>
string to_string(bitset<N> v) {
  string res = "";
  for (size_t i = 0; i < N; i++) {
    res += static_cast<char>('0' + v[i]);
  }
  return res;
}
template <typename A>
string to_string(A v) {
  bool first = true;
  string res = "{";
  for (const auto &x : v) {
    if (!first) {
      res += ", ";
    }
    first = false;
    res += to_string(x);
  }
  res += "}";
  return res;
}
template <typename A, typename B>
string to_string(pair<A, B> p) {
  return "(" + to_string(p.first) + ", " + to_string(p.second) + ")";
}
template <typename A, typename B, typename C>
string to_string(tuple<A, B, C> p) {
  return "(" + to_string(get<0>(p)) + ", " + to_string(get<1>(p)) + ", " +
         to_string(get<2>(p)) + ")";
}
template <typename A, typename B, typename C, typename D>
string to_string(tuple<A, B, C, D> p) {
  return "(" + to_string(get<0>(p)) + ", " + to_string(get<1>(p)) + ", " +
         to_string(get<2>(p)) + ", " + to_string(get<3>(p)) + ")";
}
void debug_out() { cerr << '\n'; }
template <typename Head, typename... Tail>
void debug_out(Head H, Tail... T) {
  cerr << " " << to_string(H);
  debug_out(T...);
}
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;
}
void no() {
  cout << ("NO") << '\n';
  exit(0);
}
void yes() {
  cout << ("YES") << '\n';
  exit(0);
}
const ll mod = 1e9 + 7;
const ll inf = 1e18;
const double PI = acos(-1);
struct Edge {
  ll to;
  ll cost;
  Edge(ll to, ll cost) : to(to), cost(cost) {}
  Edge() {
    to = 0;
    cost = 0;
  }
};
struct Node {
  ll distance;
  ll index;
  Node(ll d, ll i) {
    distance = d;
    index = i;
  }
  Node() {}
  bool operator<(const Node &another) const {
    return distance < another.distance;
  }
  bool operator>(const Node &another) const {
    return distance > another.distance;
  }
};
struct Dijkstra {
  vector<ll> d;
  vector<vector<Edge> > graph;
  vector<bool> done;
  void initialize(ll size) {
    d.resize(size);
    done.resize(size);
    graph.resize(size);
    reset();
  }
  void reset() {
    ll N = graph.size();
    for (ll i = (0); i < (N); ++i) {
      d[i] = inf;
      done[i] = false;
    }
  }
  ll distance(int i) {
    if ((ll)d.size() <= i) return -1;
    return d[i];
  }
  void print_graph() {
    for (ll i = (0); i < ((ll)graph.size()); ++i) {
      cout << i << " -> ";
      for (auto edge : graph[i]) {
        cout << edge.to << " ";
      }
      cout << endl;
    }
    cout << ("distance") << '\n';
    for (ll i = (0); i < ((ll)graph.size()); ++i) {
      ll d = distance(i);
      cout << i << " " << d << endl;
    }
  }
  void register_edge(ll a, ll b, ll cost) {
    auto edge = Edge(b, cost);
    graph[a].push_back(edge);
  }
  void register_edge2(ll a, ll b, ll cost) {
    register_edge(a, b, cost);
    register_edge(b, a, cost);
  }
  void calc_shortest_path(ll from = 0) {
    ll N = graph.size();
    if (from < 0 || from >= N) {
      cout << "index error" << endl;
      exit(0);
    }
    priority_queue<Node, vector<Node>, greater<Node> > que;
    auto node = Node();
    node.index = from;
    node.distance = 0;
    que.push(node);
    while (!que.empty()) {
      Node n = que.top();
      que.pop();
      if (done[n.index]) {
        continue;
      }
      done[n.index] = true;
      d[n.index] = n.distance;
      auto edge_list = graph[n.index];
      for (auto edge : edge_list) {
        if (!done[edge.to] && n.distance + edge.cost < d[edge.to]) {
          ll shorter_distance = n.distance + edge.cost;
          que.push(Node(shorter_distance, edge.to));
        }
      }
    }
  }
};
VV G;
ll dfs(ll i, ll prev) {
  if (((int)(G[i]).size()) == 1) {
    return 0;
  }
  ll max_depth = 0;
  for (ll to : G[i]) {
    if (to == prev) continue;
    ll d = dfs(to, i);
    chmax(max_depth, d);
  }
  return max_depth + 1;
}
int main() {
  cin.tie(0);
  ios::sync_with_stdio(false);
  ll N, u, v;
  cin >> N >> u >> v;
  u--;
  v--;
  G.resize(N);
  Dijkstra dij, dij2;
  dij.initialize(N);
  dij2.initialize(N);
  for (ll i = (0); i < (N - 1); ++i) {
    ll a, b;
    cin >> a >> b;
    a--;
    b--;
    G[a].push_back(b);
    G[b].push_back(a);
    dij.register_edge2(a, b, 1);
    dij2.register_edge2(a, b, 1);
  }
  dij.calc_shortest_path(u);
  ll to_aoki = dij.distance(v);
  42;
  for (ll i = (0); i < (N); ++i) {
    42;
  }
  dij2.calc_shortest_path(v);
  ll up = (to_aoki - 1) / 2;
  42;
  if (up == 0) {
    ll max_depth = 0;
    for (ll to : G[u]) {
      if (dij2.distance(to) > dij2.distance(u)) {
        ll d = dfs(to, u);
        chmax(max_depth, d);
      }
    }
    if (to_aoki == 2) {
      cout << (max_depth + 1) << '\n';
      return 0;
    } else {
      cout << (max_depth) << '\n';
      return 0;
    }
  }
  for (ll _ = (0); _ < (up); ++_) {
    for (ll to : G[u]) {
      if (dij.distance(to) > dij.distance(u)) {
        u = to;
        break;
      }
    }
  }
  ll max_depth = 0;
  42;
  for (ll to : G[u]) {
    if (dij2.distance(to) > dij2.distance(u)) {
      ll d = dfs(to, u);
      42;
      chmax(max_depth, d);
    }
  }
  ll ans = up + max_depth + 1;
  if (to_aoki % 2 == 0) {
    ans++;
  }
  cout << (ans) << '\n';
  return 0;
}
