#include <bits/stdc++.h>
#pragma GCC optimize("O3")
using namespace std;
using ll = long long;
using P = pair<int, int>;
template <class T>
using V = vector<T>;
const ll inf = (1e18);
const ll mod = 1000000007;
ll gcd(ll a, ll b) { return b ? gcd(b, a % b) : a; }
ll lcm(ll c, ll d) { return c / gcd(c, d) * d; }
struct __INIT {
  __INIT() {
    cin.tie(0);
    ios::sync_with_stdio(false);
    cout << fixed << setprecision(15);
  }
} __init;
template <class T>
bool chmax(T &a, const T &b) {
  if (a < b) {
    a = b;
    return 1;
  }
  return 0;
}
template <class T>
bool chmin(T &a, const T &b) {
  if (a > b) {
    a = b;
    return 1;
  }
  return 0;
}
template <typename G>
struct DLCA {
  const int Log;
  vector<int> dep;
  const G &g;
  vector<vector<int>> table;
  explicit DLCA(const G &g)
      : g(g), dep(g.size()), Log(32 - __builtin_clz(g.size())) {
    table.assign(Log, vector<int>(g.size(), -1));
  }
  void dfs(int idx, int par, int d) {
    table[0][idx] = par;
    dep[idx] = d;
    for (auto &to : g[idx]) {
      if (to != par) dfs(to, idx, d + 1);
    }
  }
  void build() {
    dfs(0, -1, 0);
    for (int k = 0; k < Log - 1; k++) {
      for (int i = 0; i < table[k].size(); i++) {
        if (table[k][i] == -1)
          table[k + 1][i] = -1;
        else
          table[k + 1][i] = table[k][table[k][i]];
      }
    }
  }
  int query(int u, int v) {
    if (dep[u] > dep[v]) swap(u, v);
    for (int i = Log - 1; i >= 0; i--) {
      if (dep[v] - dep[u] & (1 << i)) v = table[i][v];
    }
    if (u == v) return u;
    for (int i = Log - 1; i >= 0; i--) {
      if (table[i][u] != table[i][v]) {
        u = table[i][u];
        v = table[i][v];
      }
    }
    return table[0][u];
  }
  int dist(int u, int v) {
    int par = query(u, v);
    return abs(dep[u] - dep[par]) + abs(dep[v] - dep[par]);
  }
};
int main() {
  int n, u, v;
  cin >> n >> u >> v;
  u--;
  v--;
  V<V<int>> t(n);
  for (int i = 0; i < n - 1; i++) {
    int a, b;
    cin >> a >> b;
    t[--a].push_back(--b);
    t[b].push_back(a);
  }
  DLCA<V<V<int>>> tree(t);
  tree.build();
  int ans = 0;
  int d = tree.dist(u, v);
  for (int i = 0; i < n; i++) {
    int a = tree.query(u, i);
    int b = tree.dist(a, u), c = tree.dist(c, v);
    if (a <= b) {
      chmax(ans, d / 2);
    } else {
      chmax(ans, tree.dist(v, i) - 1);
    }
  }
  cout << ans << endl;
}
