#include <bits/stdc++.h>
using namespace std;
using ll = long long;
using vll = std::vector<ll>;
using vvll = std::vector<vll>;
using vvvll = std::vector<vvll>;
const ll INF = 1LL << 60;
const int INF_INT = 1 << 30;
template <class T>
inline bool chmax(T &a, T b) {
  if (a < b) {
    a = b;
    return true;
  }
  return false;
}
template <class T>
inline bool chmin(T &a, T b) {
  if (a > b) {
    a = b;
    return true;
  }
  return false;
}
struct mll {
  static ll MOD;
  ll val;
  mll(ll v = 0) : val(v % MOD) {
    if (val < 0) val += MOD;
  }
  mll operator-() const { return -val; }
  mll operator+(const mll &b) const { return val + b.val; }
  mll operator-(const mll &b) const { return val - b.val; }
  mll operator*(const mll &b) const { return val * b.val; }
  mll operator/(const mll &b) const { return mll(*this) /= b; }
  mll operator+(ll b) const { return *this + mll(b); }
  mll operator-(ll b) const { return *this - mll(b); }
  mll operator*(ll b) const { return *this * mll(b); }
  friend mll operator+(ll a, const mll &b) { return b + a; }
  friend mll operator-(ll a, const mll &b) { return -b + a; }
  friend mll operator*(ll a, const mll &b) { return b * a; }
  mll &operator+=(const mll &b) {
    val = (val + b.val) % MOD;
    return *this;
  }
  mll &operator-=(const mll &b) {
    val = (val + MOD - b.val) % MOD;
    return *this;
  }
  mll &operator*=(const mll &b) {
    val = (val * b.val) % MOD;
    return *this;
  }
  mll &operator/=(const mll &b) {
    ll c = b.val, d = MOD, u = 1, v = 0;
    while (d) {
      ll t = c / d;
      c -= t * d;
      swap(c, d);
      u -= t * v;
      swap(u, v);
    }
    val = val * u % MOD;
    if (val < 0) val += MOD;
    return *this;
  }
  mll &operator+=(ll b) { return *this += mll(b); }
  mll &operator-=(ll b) { return *this -= mll(b); }
  mll &operator*=(ll b) { return *this *= mll(b); }
  mll &operator/=(ll b) { return *this /= mll(b); }
  bool operator==(const mll &b) { return val == b.val; }
  bool operator!=(const mll &b) { return val != b.val; }
  bool operator==(ll b) { return *this == mll(b); }
  bool operator!=(ll b) { return *this != mll(b); }
  friend bool operator==(ll a, const mll &b) { return mll(a) == b.val; }
  friend bool operator!=(ll a, const mll &b) { return mll(a) != b.val; }
  friend ostream &operator<<(ostream &os, const mll &a) { return os << a.val; }
  friend istream &operator>>(istream &is, mll &a) { return is >> a.val; }
  static mll Combination(ll a, ll b) {
    chmin(b, a - b);
    if (b < 0) return mll(0);
    mll c = 1;
    for (ll i = (0); i <= (b - 1); i++) c *= a - i;
    for (ll i = (0); i <= (b - 1); i++) c /= i + 1;
    return c;
  }
};
using vmll = std::vector<mll>;
using vvmll = std::vector<vmll>;
using vvvmll = std::vector<vvmll>;
using vvvvmll = std::vector<vvvmll>;
struct Fast {
  Fast() {
    cin.tie(0);
    ios::sync_with_stdio(false);
    cout << fixed << setprecision(std::numeric_limits<double>::max_digits10);
  }
} fast;
ll mll::MOD = (ll)(1e9 + 7);
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(WeightedGraph<T> &g, int s) {
  const auto INF = numeric_limits<T>::max();
  vector<T> dist(g.size(), INF);
  using Pi = pair<T, int>;
  priority_queue<Pi, vector<Pi>, greater<Pi> > que;
  dist[s] = 0;
  que.emplace(dist[s], s);
  while (!que.empty()) {
    T cost;
    int idx;
    tie(cost, idx) = que.top();
    que.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;
      que.emplace(dist[e.to], e.to);
    }
  }
  return dist;
}
int main() {
  ll N;
  cin >> N;
  ll U;
  cin >> U;
  ll V;
  cin >> V;
  WeightedGraph<int> g(N);
  for (ll i = (0); i <= (N - 1 - 1); i++) {
    int a, b;
    cin >> a >> b;
    a--;
    b--;
    g[a].emplace_back(b, 1);
    g[b].emplace_back(a, 1);
  }
  auto d_from_oni = dijkstra(g, V - 1);
  auto d_from_nige = dijkstra(g, U - 1);
  ll result1 = 0;
  ll result2 = INF;
  for (ll i = (0); i <= (N - 1); i++) {
    if (d_from_oni[i] > d_from_nige[i]) {
      chmax(result1, (ll)d_from_nige[i]);
    } else if (d_from_oni[i] == d_from_nige[i]) {
      chmin(result2, (ll)d_from_nige[i]);
    } else {
      chmax(result1, (ll)d_from_oni[i]);
    }
  }
  if (result2 == INF) {
    cout << result1 << endl;
  } else {
    cout << max(result1, result2) << endl;
  }
  return 0;
}
