#pragma region template
#include <bits/stdc++.h>
using namespace std;
using ll = long long;
using ull = unsigned long long;
using ld = long double;
using vi = vector<int>;
using vvi = vector<vi>;
using vvvi = vector<vvi>;
using vll = vector<ll>;
using vvll = vector<vll>;
using vvvll = vector<vvll>;
using vld = vector<ld>;
using vvld = vector<vld>;
using vvvld = vector<vvld>;
using vs = vector<string>;
using pll = pair<ll, ll>;
using vp = vector<pll>;
template <typename T>
using pqrev = priority_queue<T, vector<T>, greater<T>>;
#define rep(i, n) for (ll i = 0, i##_end = (n); i < i##_end; i++)
#define repb(i, n) for (ll i = (n)-1; i >= 0; i--)
#define repr(i, a, b) for (ll i = (a), i##_end = (b); i < i##_end; i++)
#define reprb(i, a, b) for (ll i = (b)-1, i##_end = (a); i >= i##_end; i--)
#define ALL(a) (a).begin(), (a).end()
#define SZ(x) ((ll)(x).size())
const ll MOD = 1e9 + 7;
//const ll MOD = 998244353;
const ll INF = 1e+18;
const ld EPS = 1e-12L;
const ld PI = acos(-1.L);
inline ll GCD(ll a, ll b) { return b ? GCD(b, a % b) : a; }
inline ll LCM(ll a, ll b) { return a / GCD(a, b) * b; }
template <typename S, typename T>
inline bool chmax(S &a, const T &b) {
  if (a < b) {
    a = b;
    return 1;
  }
  return 0;
}
template <typename S, typename T>
inline bool chmin(S &a, const T &b) {
  if (b < a) {
    a = b;
    return 1;
  }
  return 0;
}
#ifdef OJ_LOCAL
#include "dump.hpp"
#else
#define dump(...) ((void)0)
#endif
template <typename T>
bool print_(const T &a) {
  cout << a;
  return true;
}
template <typename T>
bool print_(const vector<T> &vec) {
  for (auto &a : vec) {
    cout << a;
    if (&a != &vec.back()) {
      cout << " ";
    }
  }
  return false;
}
template <typename T>
bool print_(const vector<vector<T>> &vv) {
  for (auto &v : vv) {
    for (auto &a : v) {
      cout << a;
      if (&a != &v.back()) {
        cout << " ";
      }
    }
    if (&v != &vv.back()) {
      cout << "\n";
    }
  }
  return false;
}
void print() { cout << "\n"; }
template <typename Head, typename... Tail>
void print(Head &&head, Tail &&... tail) {
  bool f = print_(head);
  if (sizeof...(tail) != 0) {
    cout << (f ? " " : "\n");
  }
  print(forward<Tail>(tail)...);
}
#pragma endregion


vvll g;

int main() {
  cin.tie(0);
  ios::sync_with_stdio(false);
  cout << fixed << setprecision(15);
  ll n, u, v;
  cin >> n >> u >> v;
  g.resize(n+1, vll());
  rep(i, n-1){
    ll a, b;
    cin >> a >> b;
    g[a].emplace_back(b);
    g[b].emplace_back(a);
  }
  vector<bool> visited(n+1, false);
  ll distu = 1;
  vi stat(n+1, 0);
  vi stat2(n+1, 0);
  vi dist(n+1, 0);
  auto dfs = [&](auto& Self, int node, int d) -> bool{
    visited[node] = true;
    if(node == u) distu = d;
    bool f = 0;
    if(node == u) f = 1;
    for(auto&& e: g[node]){
      if(!visited[e]){
        if(Self(Self, e, d+1)) f = 1;
      }
    }
    stat[node] = f;
    dist[node] = d;
    return f;
  };
  dfs(dfs, v, 0);
  stat2 = stat;
  repr(i, 1, n+1){
    if(dist[i] < distu/2+1) stat2[i] = 0;
  }
  
  vector<bool> visited2(n+1, false);
  ll ans = -1;
  auto dfs2 = [&](auto& Self, int node, bool f) -> void{
    visited2[node] = true;
    if(stat2[node]) f = 0;
    if(f){
      for(auto&& e: g[node]){
        if(!visited2[e] && stat[node]){
          Self(Self, e, 1);
        }
      }
    }else{
      for(auto&& e: g[node]){
        if(!visited2[e]){
          Self(Self, e, 0);
        }
      }
    }
    chmax(ans, dist[node]-1);
  };
  dfs2(dfs2, v, 1);
  dump(distu);
  dump(stat);
  dump(stat2);
  dump(dist);
  print(ans);
}


Measured memory_bytes: 13056