#include <bits/stdc++.h>
using namespace std;
void __print(int x) { cerr << x; }
void __print(long x) { cerr << x; }
void __print(float x) { cerr << x; }
void __print(double x) { cerr << x; }
void __print(unsigned x) { cerr << x; }
void __print(long long x) { cerr << x; }
void __print(long double x) { cerr << x; }
void __print(unsigned long x) { cerr << x; }
void __print(unsigned long long x) { cerr << x; }
void __print(char x) { cerr << '\'' << x << '\''; }
void __print(bool x) { cerr << (x ? "true" : "false"); }
void __print(const char *x) { cerr << '\"' << x << '\"'; }
void __print(const string &x) { cerr << '\"' << x << '\"'; }
template <typename T, typename V>
void __print(const pair<T, V> &x) {
  cerr << '{';
  __print(x.first);
  cerr << ',';
  __print(x.second);
  cerr << '}';
}
template <typename T>
void __print(const T &x) {
  int f = 0;
  cerr << '{';
  for (auto &i : x) cerr << (f++ ? "," : ""), __print(i);
  cerr << "}";
}
void _print() { cerr << "]\n"; }
template <typename T, typename... V>
void _print(T t, V... v) {
  __print(t);
  if (sizeof...(v)) cerr << ", ";
  _print(v...);
}
struct _ {
  ios_base::Init i;
  _() { ios_base::sync_with_stdio(false), cin.tie(NULL), cout.tie(NULL); }
} _;
int dx[] = {-1, 0, 1, 0, -1, -1, 1, 1};
int dy[] = {0, 1, 0, -1, -1, 1, 1, -1};
int n, src, dest;
vector<vector<int>> gr;
vector<vector<int>> dist;
void bfs(int str, int type) {
  dist[str][type] = 0;
  queue<int> q;
  q.push(str);
  while (!q.empty()) {
    int parent = q.front();
    q.pop();
    for (auto child : gr[parent]) {
      if (dist[child][type] == INT_MAX) {
        dist[child][type] = dist[parent][type] + 1;
        q.push(child);
      }
    }
  }
}
int dfs(int curr, int par) {
  int mx_dep = 0;
  for (auto child : gr[curr]) {
    if (child == par or dist[child][0] >= dist[child][1]) continue;
    mx_dep = max(mx_dep, dfs(child, curr));
  }
  return mx_dep + 1;
}
int main() {
  cin >> n >> src >> dest;
  gr.resize(n + 1);
  dist.resize(n + 1, vector<int>(2, INT_MAX));
  for (int i = 1; i <= n - 1; i++) {
    int u, v;
    cin >> u >> v;
    gr[u].push_back(v);
    gr[v].push_back(u);
  }
  if (src == dest) {
    cout << 0;
    return 0;
  }
  bfs(src, 0);
  bfs(dest, 0);
  cout << dfs(dest, dest) - 2;
  return 0;
}
