#define STOPIT
#include <bits/stdc++.h>
#define loop(n) for (lint ngtkana_is_a_genius = 0; ngtkana_is_a_genius < lint(n); ngtkana_is_a_genius++)
#define rep(i, begin, end) for (lint i = lint(begin); (i) < lint(end); i++)
#define all(v) v.begin(), v.end()
#define rand(l, r) std::uniform_int_distribution<>(l, r)(mt)
using lint = long long;
auto mt = std::mt19937_64(std::random_device{}());
auto cmn = [](auto& a, auto b){ if (a > b) {a = b; return true;} return false; };
auto cmx = [](auto& a, auto b){ if (a < b) {a = b; return true;} return false; };
void debug_impl() { std::cerr << std::endl; }
template <typename Head, typename... Tail>
void debug_impl(Head head, Tail... tail) { std::cerr << " " << head; debug_impl(tail...); }
#ifndef STOPIT
#define debug(...)\
  do {\
    std::cerr << std::boolalpha << "[" << #__VA_ARGS__ << "]:";\
    debug_impl(__VA_ARGS__);\
    std::cerr << std::noboolalpha;\
  } while (false)
#else
#define debug(...) {}
#endif

template < typename Container, typename Value = typename Container::value_type, std::enable_if_t<!std::is_same< Container, std::string >::value, std::nullptr_t> = nullptr>
std::istream& operator>> (std::istream& is, Container& v)
  { for (auto & x : v) { is >> x; } return is; }

template < typename Container, typename Value = typename Container::value_type, std::enable_if_t<!std::is_same< Container, std::string >::value, std::nullptr_t> = nullptr >
std::ostream& operator<< (std::ostream& os, Container const& v) {
 os << "{";
  for (auto it = v.begin(); it != v.end(); it++)
    {os << (it != v.begin() ? "," : "") << *it;}
  return os << "}";
}

int main() {
  std::cin.tie(0); std::cin.sync_with_stdio(false);
  int n, s, t;
  std::cin >> n >> s >> t;
  s--,t--;
  int root = t;
  std::vector<std::vector<int>> graph(n);
  loop(n-1) {
    int u, v; std::cin >> u >> v;
    u--,v--;
    graph.at(u).emplace_back(v);
    graph.at(v).emplace_back(u);
  }
  debug(graph);

  int D = 0;
  std::vector<int> h(n,-1);
  auto dfs = [&] (auto&& dfs, int x, int p, int d) -> void {
    if (x == s) D = d;
    for (auto&& y : graph.at(x)) {
      if (y == p) continue;
      dfs(dfs,y,x,d+1);
      cmx(h.at(x), h.at(y));
    }
    h.at(x)++;
  };
  dfs(dfs,root,root,0);
  debug(D);

  int ans = D / 2;
  auto efs = [&] (auto&& efs, int x, int p, int d) -> bool {
    bool ret = x == s;
    for (auto&& y : graph.at(x)) {
      if (y == p) continue;
      ret |= efs(efs,y,x,d+1);
    }
    if (ret == true && D / 2 < d) {
      lint tmp = D - d + h.at(x);
      debug(x,d,h.at(x),D,tmp);
      cmx(ans,tmp);
      debug(ans,tmp);
    }
    return ret;
  };
  efs(efs,root,root,0);

  // debug(h);
  std::cout << ans << std::endl;
  return 0;
}