#include <bits/stdc++.h>
using namespace std;
template <class T>
struct Dijkstra {
  const T inf = numeric_limits<T>::max() / 2 - 1;
  vector<T> dist;
  vector<int> path;
  Dijkstra() {}
  Dijkstra(int n) {
    graph.resize(n);
    dist.resize(n);
    prev.resize(n);
  }
  void add_edge(int from, int to, T cost) {
    graph[from].emplace_back(to, cost);
  }
  void get_distance(int from) {
    priority_queue<pair<T, int>, vector<pair<T, int>>, greater<pair<T, int>>>
        pq;
    fill(dist.begin(), dist.end(), inf);
    fill(prev.begin(), prev.end(), -1);
    dist[from] = 0;
    pq.push(make_pair(0, from));
    while (!pq.empty()) {
      pair<T, int> p = pq.top();
      pq.pop();
      if (dist[p.second] < p.first) continue;
      for (auto e : graph[p.second]) {
        if (dist[e.first] > (dist[p.second] + e.second)) {
          dist[e.first] = dist[p.second] + e.second;
          prev[e.first] = p.second;
          pq.push(make_pair(dist[e.first], e.first));
        }
      }
    }
  }
  void get_path(int to) {
    path = vector<int>(0);
    for (; to != -1; to = prev[to]) {
      path.push_back(to);
    }
    reverse(path.begin(), path.end());
  }

 private:
  vector<vector<pair<int, T>>> graph;
  vector<T> prev;
};
int main() {
  cin.tie(0);
  ios::sync_with_stdio(false);
  long long n, u, v;
  cin >> n >> u >> v;
  Dijkstra<long long> di(n);
  for (long long i = 0, i_len = (n - 1); i < i_len; i++) {
    long long a, b;
    cin >> a >> b;
    a--;
    b--;
    di.add_edge(a, b, 1);
    di.add_edge(b, a, 1);
  }
  di.get_distance(0);
  long long mv = 0, mi = -1;
  for (long long i = 0, i_len = (n); i < i_len; i++) {
    if (di.dist[i] > mv) {
      mv = di.dist[i];
      mi = i;
    }
  }
  di.get_distance(mi);
  mv = 0, mi = -1;
  for (long long i = 0, i_len = (n); i < i_len; i++) {
    if (di.dist[i] > mv) {
      mv = di.dist[i];
      mi = i;
    }
  }
  cout << (mv - 1) << endl;
  return 0;
}
