#include <bits/stdc++.h>
using namespace std;
int main() {
  ios::sync_with_stdio(false);
  cin.tie(0);
  int n, u, v;
  cin >> n >> u >> v;
  u--;
  v--;
  vector<vector<int>> to(n);
  for (int i = 0; i < n - 1; i++) {
    int x, y;
    cin >> x >> y;
    x--;
    y--;
    to[x].push_back(y);
    to[y].push_back(x);
  }
  function<void(int, int, vector<int> &)> dfs = [&](int u, int p,
                                                    vector<int> &d) {
    for (int v : to[u]) {
      if (v == p) continue;
      d[v] = d[u] + 1;
      dfs(v, u, d);
    }
  };
  vector<int> du(n, 0), dv(n, 0);
  dfs(u, -1, du);
  dfs(v, -1, dv);
  int ans = du[v] / 2;
  for (int i = 0; i < n; i++) {
    if (du[i] < dv[i]) {
      ans = max(ans, du[i]);
    }
  }
  cout << ans << endl;
  return 0;
}
