#include <bits/stdc++.h>
#pragma GCC optimize("unroll-loops,no-stack-protector")
#pragma GCC target("sse,sse2,ssse3,sse4,popcnt,abm,mmx,avx,tune=native")
using namespace std;
long long gcd(long long a, long long b) { return (!b ? a : gcd(b, a % b)); }
long long lcm(long long a, long long b) { return ((a * b) / gcd(a, b)); }
bool cmp(int a, int b) { return a > b; }
const long long mod = 1e9 + 7;
const int INF32 = 1 << 30;
const long long INF64 = 1LL << 60;
const long double pi = 3.141592653589793;
long long modpow(long long n, long long k, long long mod) {
  if (k == 0) return 1;
  long long r = modpow(n * n % mod, k >> 1, mod);
  if (k & 1) r = r * n % mod;
  return r;
}
vector<vector<int>> adj(1e5 + 10);
void dfs(int v, int prev, vector<int>& depth) {
  for (auto vi : adj[v]) {
    if (vi == prev) continue;
    depth[vi] = depth[v] + 1;
    dfs(vi, v, depth);
  }
}
void solve() {
  int n, a, b;
  cin >> n >> a >> b;
  for (int i = 0; i < n - 1; i++) {
    int v, u;
    cin >> v >> u;
    v--;
    u--;
    adj[v].push_back(u);
    adj[u].push_back(v);
  }
  vector<int> depth1(1e5 + 10, 0);
  vector<int> depth2(1e5 + 10, 0);
  dfs(a, -1, depth1);
  dfs(b, -1, depth2);
  int ans = 0;
  for (int i = 0; i < n; i++) {
    if (depth1[i] <= depth2[i]) ans = max(ans, depth2[i]);
  }
  cout << ans;
}
int main() {
  ios_base::sync_with_stdio(false);
  cin.tie(NULL);
  cout.tie(NULL);
  solve();
  return 0;
}
