#include <bits/stdc++.h>
using namespace std;
const int inf = 1e9 + 7;
const long long INF = 1e18;
int mod = 1000000007;
template <typename T1, typename T2>
inline bool chmin(T1& a, T2 b) {
  if (a > b) {
    a = b;
    return 1;
  }
  return 0;
}
template <typename T1, typename T2>
inline bool chmax(T1& a, T2 b) {
  if (a < b) {
    a = b;
    return 1;
  }
  return 0;
}
template <typename T>
T gcd(T a, T b) {
  if (b == 0) return a;
  return gcd(b, a % b);
}
template <typename T>
T lcm(T a, T b) {
  return a / gcd(a, b) * b;
}
template <typename T>
T pow(T a, int b) {
  return b ? pow(a * a, b / 2) * (b % 2 ? a : 1) : 1;
}
long long modpow(long long a, long long b, long long _mod) {
  return b ? modpow(a * a % _mod, b / 2, _mod) * (b % 2 ? a : 1) % _mod : 1;
}
template <class T>
ostream& operator<<(ostream& os, const vector<T>& vec) {
  for (auto& vi : vec) os << vi << " ";
  return os;
}
template <class T, class U>
ostream& operator<<(ostream& os, const pair<T, U>& p) {
  os << p.first << " " << p.second;
  return os;
}
template <typename T>
inline istream& operator>>(istream& is, vector<T>& v) {
  for (int j = 0; j < ((int)v.size()); ++j) is >> v[j];
  return is;
}
template <class T>
inline void add(T& a, int b) {
  a += b;
  if (a >= mod) a -= mod;
}
void solve();
int main() {
  ios::sync_with_stdio(false);
  cin.tie(0);
  cout.tie(0);
  cout << fixed << setprecision(10);
  int T;
  T = 1;
  while (T--) {
    solve();
  }
}
int n;
int u, v;
const int N = 100005;
vector<vector<int> > G(N);
vector<int> d(N, inf);
void dfs(int v, int p) {
  for (int nv : G[v]) {
    if (nv == p) continue;
    if (chmin(d[nv], d[v] + 1)) dfs(nv, v);
  }
}
void solve() {
  cin >> n;
  cin >> u >> v;
  u--;
  v--;
  for (int i = 0; i < (n - 1); ++i) {
    int a, b;
    cin >> a >> b;
    a--;
    b--;
    G[a].emplace_back(b);
    G[b].emplace_back(a);
  }
  d[u] = 0;
  dfs(u, -1);
  vector<int> d2 = d;
  d = vector<int>(N, inf);
  d[v] = 0;
  dfs(v, -1);
  pair<int, int> ma = pair<int, int>(0, -1);
  for (int i = 0; i < (n); ++i) {
    if (d2[i] < d[i] && chmax(ma.first, d[i])) {
      ma.second = i;
    }
  }
  cout << max(0, (v - ma.first >= 0 ? v - ma.first : -(v - ma.first)) - 1)
       << endl;
}
