#include <iostream>
#include <bits/stdc++.h>

#include <ext/pb_ds/assoc_container.hpp>
#include <ext/pb_ds/tree_policy.hpp>

#define rep(i, n) for (int i = 0; i < (int)(n); i++)
#define rep_r(i, n) for (int i = n - 1; i >= 0; i--)
#define rep1(i, n) for (int i = 1; i <= (int)(n); i++)
#define REP(i, n) for (int i = 0, i##_len = (n); i < i##_len; ++i)
#define all(x) (x).begin(), (x).end()
#define SZ(x) ((ll)(x).size())
#define bit(n) (1LL << (n))
#define UNIQUE(v) v.erase(unique(v.begin(), v.end()), v.end());

#define INF bit(60)
#define pb push_back
#define mod 1000000007

using namespace std;
using uif = uint_fast64_t;
using ll = long long int;
using tTree = __gnu_pbds::tree<ll, __gnu_pbds::null_type, less<ll>, __gnu_pbds::rb_tree_tag, __gnu_pbds::tree_order_statistics_node_update>;

ll dx[4] = {1, 0, -1, 0};
ll dy[4] = {0, 1, 0, -1};

#define FACSIZE 200002

ll invfac[FACSIZE];
ll fac[FACSIZE];

template <class T>
bool chmax(T &, const T &);
template <class T>
bool chmin(T &, const T &);
ll gcd(ll, ll);
ll powLL(ll x, ll y);

ll mod_pow(ll, ll);
ll mod_add(ll, ll);
ll mod_mul(ll, ll);
ll mod_div(ll, ll);
ll comb(ll, ll);
void make_fact(ll);
void make_invfact(ll);
void fact_init();

ll n, u, v;

struct Tree
{

  ll val;
  ll parent;
  Tree *PARENT;
  unordered_set<Tree *> children;
};

Tree *U;
vector<vector<ll>> edges;
vector<ll> depths;
vector<ll> fromU;
void processInput();
Tree *initRoot(ll);
void make_tree(Tree *);
void bfs(Tree *);
void bfsU(Tree *);
void dfs(Tree *);

ll deepest(Tree *node)
{

  ll ans = node->val;
  queue<Tree *> que;
  que.push(node);

  while (!que.empty())
  {
    auto q = que.front();
    que.pop();
    if (depths[q->val] > depths[ans])
      ans = q->val;
    for (auto no : q->children)
    {
      que.push(no);
    }
  }

  return ans;
}

int main(void)
{
  processInput();
  auto root = initRoot(v);
  //cout << root->val << endl;
  make_tree(root);
  auto rootU = initRoot(u);
  //cout << root->val << endl;
  make_tree(rootU);
  depths.resize(n + 1, 0);
  fromU.resize(n + 1, 0);
  bfs(root);
  bfsU(rootU);

  ll ans = 0;
  /*
  for (auto x : depths)
    cout << x << " ";
  cout << endl;
  for (auto x : fromU)
    cout << x << " ";
  cout << endl;
  */

  auto cur = root;
  queue<Tree *> que;
  que.push(rootU);

  vector<ll> cand;

  while (!que.empty())
  {
    auto q = que.front();
    que.pop();
    //cout << q->val << endl;
    if ((q->children).size() == 0)
      cand.push_back(q->val);
    for (auto c : q->children)
    {
      if (fromU[c->val] < depths[c->val])
        que.push(c);
    }
  }
  /*
  for (auto x : cand)
    cout << x << endl;
    */
  if (cand.size() == 0)
  {
    cout << depths[u] - 1 << endl;
    return 0;
  }
  ans = u;
  for (auto x : cand)
  {
    if (depths[x] > depths[ans])
    {
      ans = x;
    }
  }

  cout << depths[ans] - 1 << endl;

  return 0;
}

ll mod_pow(ll x, ll r)
{
  if (r == 0)
    return 1;
  else if (r == 1)
    return x % mod;
  else if (r % 2 == 0)
  {
    ll t = mod_pow(x, r / 2) % mod;
    return mod_mul(t, t);
  }
  else
  {
    ll t = mod_pow(x, r / 2) % mod;
    ll k = mod_mul(t, t);
    return (k % mod) * (x % mod);
  }
}
ll mod_add(ll a, ll b)
{
  return ((a % mod) + (b % mod)) % mod;
}

ll mod_mul(ll a, ll b)
{
  return ((a % mod) * (b % mod)) % mod;
}

ll mod_div(ll a, ll b)
{
  return mod_mul(a, mod_pow(b, mod - 2));
}

void fact_init()
{
  make_fact(FACSIZE - 1);
  make_invfact(FACSIZE);
}

void make_fact(ll n)
{
  fac[0] = 1;
  rep(i, n)
  {
    fac[i + 1] = mod_mul(fac[i], i + 1);
  }
}

void make_invfact(ll n)
{
  invfac[n] = mod_pow(fac[n], mod - 2);
  for (int i = n - 1; i >= 0; i--)
  {
    invfac[i] = mod_mul(invfac[i + 1], i + 1);
  }
}

ll comb(ll n, ll r)
{

  return mod_mul(mod_mul(fac[n], invfac[r]), invfac[n - r]);
}
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 (b < a)
  {
    a = b;
    return 1;
  }
  return 0;
}

ll qp(ll a, ll b)
{
  ll ans = 1LL;
  do
  {
    if (b & 1LL)
      ans = 1LL * mod_mul(ans, a) % mod;
    a = 1LL * mod_mul(a, a) % mod;
  } while (b >>= 1LL);
  return ans;
}
ll qp(ll a, ll b, ll mo)
{
  ll ans = 1LL;
  do
  {
    if (b & 1LL)
      ans = 1LL * (ans % mo) * (a % mo);
    a = 1LL * (a % mo) * (a % mo);
  } while (b >>= 1LL);
  return ans;
}
ll gcd(ll a, ll b)
{
  return b ? gcd(b, a % b) : a;
}
ll powLL(ll x, ll y)
{
  ll ans = 1LL;
  for (ll i = 0LL; i < y; i++)
    ans *= x;
  return ans;
}

void processInput()
{

  cin >> n >> u >> v;

  edges.resize(n + 1);

  rep(i, n - 1)
  {
    ll a, b;
    cin >> a >> b;
    edges[a].push_back(b);
    edges[b].push_back(a);
  }
}

Tree *initRoot(ll x)
{
  Tree *root = new Tree;
  root->parent = 0;
  root->val = x;

  return root;
}

void make_tree(Tree *root)
{

  queue<Tree *> que;
  que.push(root);

  //for(auto x:edges[]){}
  while (!que.empty())
  {
    auto node = que.front();
    que.pop();

    for (auto nextVal : edges[node->val])
    {
      if (nextVal != node->parent)
      {
        auto nextNode = new Tree;
        nextNode->val = nextVal;
        nextNode->parent = node->val;
        nextNode->PARENT = node;
        node->children.insert(nextNode);
        que.push(nextNode);
      }
    }
  }
}

void bfs(Tree *root)
{
  using pa = pair<Tree *, ll>;
  queue<pa> que;
  que.push(make_pair(root, 0));

  while (!que.empty())
  {
    auto node = que.front().first;
    auto depth = que.front().second;
    if (node->val == u)
      U = node;
    que.pop();
    //cout << node->val << "," << depth << endl;
    depths[node->val] = depth;
    for (auto n : node->children)
    {
      que.push(make_pair(n, depth + 1));
    }
  }
}

void bfsU(Tree *root)
{
  using pa = pair<Tree *, ll>;
  queue<pa> que;
  que.push(make_pair(root, 0));

  while (!que.empty())
  {
    auto node = que.front().first;
    auto depth = que.front().second;
    if (node->val == u)
      U = node;
    que.pop();
    //cout << node->val << "," << depth << endl;
    fromU[node->val] = depth;
    for (auto n : node->children)
    {
      que.push(make_pair(n, depth + 1));
    }
  }
}

Measured memory_bytes: 38528