//#define NDEBUG
#include <algorithm>
#include <cstddef>
#include <cstdint>
#include <iostream>
#include <utility>
#include <vector>

namespace n91 {

using i8 = std::int_fast8_t;
using i32 = std::int_fast32_t;
using i64 = std::int_fast64_t;
using u8 = std::uint_fast8_t;
using u32 = std::uint_fast32_t;
using u64 = std::uint_fast64_t;
using isize = std::ptrdiff_t;
using usize = std::size_t;

struct rep {
  struct itr {
    usize i;
    constexpr itr(const usize i) noexcept : i(i) {}
    void operator++() noexcept { ++i; }
    constexpr usize operator*() const noexcept { return i; }
    constexpr bool operator!=(const itr x) const noexcept { return i != x.i; }
  };
  const itr f, l;
  constexpr rep(const usize f, const usize l) noexcept
      : f(std::min(f, l)), l(l) {}
  constexpr auto begin() const noexcept { return f; }
  constexpr auto end() const noexcept { return l; }
};
struct revrep {
  struct itr {
    usize i;
    constexpr itr(const usize i) noexcept : i(i) {}
    void operator++() noexcept { --i; }
    constexpr usize operator*() const noexcept { return i; }
    constexpr bool operator!=(const itr x) const noexcept { return i != x.i; }
  };
  const itr f, l;
  constexpr revrep(const usize f, const usize l) noexcept
      : f(l - 1), l(std::min(f, l) - 1) {}
  constexpr auto begin() const noexcept { return f; }
  constexpr auto end() const noexcept { return l; }
};
template <class T> auto md_vec(const usize n, const T &value) {
  return std::vector<T>(n, value);
}
template <class... Args> auto md_vec(const usize n, Args... args) {
  return std::vector<decltype(md_vec(args...))>(n, md_vec(args...));
}
template <class T> constexpr T difference(const T &a, const T &b) noexcept {
  if (a < b) {
    return b - a;
  } else {
    return a - b;
  }
}
template <class T> void chmin(T &a, const T &b) noexcept {
  if (b < a) {
    a = b;
  }
}
template <class T> void chmax(T &a, const T &b) noexcept {
  if (a < b) {
    a = b;
  }
}
template <class F> class fix_point : private F {
public:
  explicit constexpr fix_point(F &&f) : F(std::forward<F>(f)) {}

  template <class... Args>
  constexpr decltype(auto) operator()(Args &&... args) const {
    return F::operator()(*this, std::forward<Args>(args)...);
  }
};
template <class F> constexpr decltype(auto) make_fix(F &&f) {
  return fix_point<F>(std::forward<F>(f));
}
template <class T> T scan() {
  T ret;
  std::cin >> ret;
  return ret;
}

} // namespace n91

#include <algorithm>
#include <iostream>
#include <string>
#include <utility>
#include <vector>

namespace n91 {

void main_() {
  /*
  std::ios::sync_with_stdio(false);
  std::cin.tie(nullptr);
  //*/
  const usize n = scan<usize>();
  const usize u = scan<usize>() - 1;
  const usize v = scan<usize>() - 1;
  auto g = md_vec(n, 0, usize());
  for (const usize i : rep(0, n - 1)) {
    const usize a = scan<usize>() - 1;
    const usize b = scan<usize>() - 1;
    g[a].push_back(b);
    g[b].push_back(a);
  }
  std::vector<usize> du(n), dv(n);
  const auto dfs = make_fix([&](const auto &dfs, const usize x, const usize p,
                                const usize d, std::vector<usize> &da) -> void {
    da[x] = d;
    for (const auto e : g[x]) {
      if (e != p) {
        dfs(e, x, d + 1, da);
      }
    }
  });
  dfs(u, n, 0, du);
  dfs(v, n, 0, dv);
  usize ans = 0;
  for (const usize i : rep(0, n)) {
    if (du[i] < dv[i]) {
      chmax(ans, dv[i] - 1);
    }
  }
  std::cout << ans << std::endl;
}

} // namespace n91

int main() {
  n91::main_();
  return 0;
}


Measured memory_bytes: 11008