#include <bits/stdc++.h>
using namespace std;
template <typename T, typename S>
ostream &operator<<(ostream &os, const pair<T, S> &p) {
  return os << "(" << p.first << ", " << p.second << ")";
}
template <typename C, typename T = decay<decltype(*begin(declval<C>()))>,
          typename enable_if<!is_same<C, string>::value>::type * = nullptr>
ostream &operator<<(ostream &os, const C &c) {
  bool f = true;
  os << "[";
  for (const auto &x : c) {
    if (!f) os << ", ";
    f = false;
    os << x;
  }
  return os << "]";
}
template <typename T>
void debug(string s, T x) {
  cerr << s << " = " << x << "\n";
}
template <typename T, typename... Args>
void debug(string s, T x, Args... args) {
  cerr << s.substr(0, s.find(',')) << " = " << x << " | ";
  debug(s.substr(s.find(',') + 2), args...);
}
int distA[100005], distB[100005];
vector<int> adj[100005];
void dfs(int u, int p, int *dist) {
  for (int v : adj[u])
    if (v != p) {
      dist[v] = dist[u] + 1;
      dfs(v, u, dist);
    }
}
int main() {
  ios_base::sync_with_stdio(false);
  cin.tie(NULL);
  int n, a, b;
  cin >> n >> a >> b;
  a--, b--;
  for (int i = 0; i < n - 1; i++) {
    int u, v;
    cin >> u >> v;
    u--, v--;
    adj[v].push_back(u);
    adj[u].push_back(v);
  }
  dfs(a, -1, distA);
  dfs(b, -1, distB);
  int ret = 0;
  for (int i = 0; i < n; i++)
    if (distA[i] < distB[i])
      ret = max(ret, distA[i] + (distB[i] - distA[i]) / 2);
  cout << ret << "\n";
  return 0;
}
