#include <bits/stdc++.h>
using namespace std;
using ll = long long;
using ld = long double;
using pll = pair<ll, ll>;
using plll = pair<ll, pll>;
using graph = vector<vector<int>>;
using wgraph = vector<vector<pll>>;
const ll MOD = 1000000007;
const int dx[4] = {1, 0, -1, 0};
const int dy[4] = {0, 1, 0, -1};
ll abs1(ll a) {
  if (a >= 0)
    return a;
  else
    return -a;
}
ll pow1(ll a, ll b, ll p = LLONG_MAX) {
  ll ret = 1;
  while (b != 0) {
    if ((b & 1) == 1) {
      ret *= a;
    }
    a *= a;
    if (a >= p) a %= p;
    b = b >> 1;
  }
  return ret;
}
int digit(ll a, ll b) {
  int i = 0;
  while (a != 0) {
    a /= b;
    i++;
  }
  return i;
}
ll gcd1(ll a, ll b) {
  if (a == 0 && b == 0) return 0;
  ll n;
  while (a % b != 0) {
    n = a % b;
    a = b;
    b = n;
  }
  return b;
}
ll solveLinear(ll n, ll m, ll c) {
  if (n == 0 && m == 0) return 0;
  ll a[100], b[100];
  a[0] = n;
  b[0] = m;
  int i = 1;
  while (b[i - 1] != 0) {
    a[i] = b[i - 1];
    b[i] = a[i - 1] % b[i - 1];
    i++;
  }
  n = i;
  if (c % a[n - 1] != 0) {
    return 0;
  }
  ll x = c / a[n - 1], y = 0;
  for (i = 1; i < n; i++) {
    ll cnt = x;
    x = y;
    y = cnt - (a[n - i - 1] / b[n - i - 1]) * y;
  }
  return x;
}
ll modRev(ll a, ll p = MOD) {
  a %= p;
  ll n = solveLinear(a, p, 1);
  n %= p;
  if (n < 0) return n + p;
  return n;
}
ll fact(ll n, ll p = MOD) {
  if (n < 0) return 0;
  int i;
  ll ret = 1;
  for (i = 0; i < n; i++) {
    ret = ret * (i + 1);
    if (ret >= p) ret %= p;
  }
  return ret;
}
ll perm(ll n, ll k, ll p = MOD) {
  if (n < 0 || k < 0 || n < k) return 0;
  int i;
  ll ret = 1;
  for (i = 0; i < k; i++) {
    ret = ret * (n - i);
    if (ret >= p) ret %= p;
  }
  return ret;
}
ll binom(ll n, ll k, ll p = MOD) {
  if (n < 0 || k < 0 || n < k) return 0;
  int i;
  ll ret = 1, fact = 1;
  for (i = 0; i < k; i++) {
    ret = ret * (n - i);
    fact *= i + 1;
    if (ret >= p) ret %= p;
    if (fact >= p) fact %= p;
  }
  return (ret * modRev(fact)) % p;
}
void pushHeap(vector<ll>& h, int n) {
  int i = h.size();
  h.push_back(n);
  while (i != 0 && h[(i - 1) / 2] < h[i]) {
    swap(h[i], h[(i - 1) / 2]);
    i = (i - 1) / 2;
  }
  h[i] = n;
}
ll popHeap(vector<ll>& h) {
  int n = h.size() - 1;
  if (n == -1) return 0;
  ll i, ret = h[0];
  swap(h[0], h[n]);
  h.pop_back();
  for (i = 0; i < n;) {
    int j = 2 * i + 1;
    if (j >= n) break;
    if (j != n - 1 && h[j] < h[j + 1]) j++;
    if (h[i] < h[j]) {
      swap(h[i], h[j]);
    }
    i = j;
  }
  return ret;
}
int bis(vector<int> v, int key) {
  int left = -1, right = v.size();
  while (abs1(right - left) > 1) {
    int mid = left + (right - left) / 2;
    if (v[mid] <= key)
      left = mid;
    else
      right = mid;
  }
  return left;
}
vector<int> aoki, taka;
void dfsa(const graph& g, int v, int p, int d) {
  aoki[v] = d;
  for (auto i : g[v]) {
    if (i == p) continue;
    dfsa(g, i, v, d + 1);
  }
}
void dfst(const graph& g, int v, int p, int d) {
  taka[v] = d;
  for (auto i : g[v]) {
    if (i == p) continue;
    if (aoki[i] < d + 1) {
      taka[i] = d;
      continue;
    }
    if (aoki[i] == d + 1) {
      taka[i] = d + 1;
      continue;
    }
    dfst(g, i, v, d + 1);
  }
}
int main() {
  int i, j, k, cnt = 0;
  int n, u, v;
  cin >> n >> u >> v;
  graph g(n);
  for (i = 0; i < n - 1; i++) {
    int a, b;
    cin >> a >> b;
    a--;
    b--;
    g[a].push_back(b);
    g[b].push_back(a);
  }
  aoki.assign(n, 0);
  taka.assign(n, 0);
  dfsa(g, v - 1, -1, 0);
  dfst(g, u - 1, -1, 0);
  int ans = 0;
  for (i = 0; i < n; i++) {
    ans = max(ans, taka[i]);
  }
  cout << ans << endl;
}
