#include <bits/stdc++.h>
using namespace std;

#define int long long
//typedef pair<int, int> P;
#define mod 1000000007
#define INF (1LL<<60)
 
#define rep(i,n) for(int i=0, i##_len=(n); i<i##_len; ++i)
#define repi(itr, ds) for (auto itr = ds.begin(); itr != ds.end(); itr++)
#define YES puts("YES")
#define Yes puts("Yes")
#define NO  puts("NO")
#define No  puts("No")
int gcd(int a, int b) { return b ? gcd(b, a % b) : a; }
 
const int dx[4] = { 1, 0, -1, 0 };
const int dy[4] = { 0, 1, 0, -1 };
 
 
#if 1
//-------------
// DUMPマクロ
// https://www.creativ.xyz/dump-cpp-652/
 
// vector
template <typename T>
istream& operator>>(istream& is, vector<T>& vec) {
    for (T& x : vec) is >> x;
    return is;
}
// pair
template <typename T, typename U>
ostream& operator<<(ostream& os, pair<T, U>& pair_var) {
    os << "(" << pair_var.first << ", " << pair_var.second << ")";
    return os;
}
// vector
template <typename T>
ostream& operator<<(ostream& os, const vector<T>& vec) {
    os << "{";
    for (int i = 0; i < vec.size(); i++) {
        os << vec[i] << (i + 1 == vec.size() ? "" : ", ");
    }
    os << "}";
    return os;
}
// map
template <typename T, typename U>
ostream& operator<<(ostream& os, map<T, U>& map_var) {
    os << "{";
    repi(itr, map_var) {
        os << *itr;
        itr++;
        if (itr != map_var.end()) os << ", ";
        itr--;
    }
    os << "}";
    return os;
}
// set
template <typename T>
ostream& operator<<(ostream& os, set<T>& set_var) {
    os << "{";
    repi(itr, set_var) {
        os << *itr;
        itr++;
        if (itr != set_var.end()) os << ", ";
        itr--;
    }
    os << "}";
    return os;
}
 
#define DUMPOUT cerr
 
void dump_func() {
    DUMPOUT << endl;
}
template <class Head, class... Tail>
void dump_func(Head&& head, Tail&& ... tail) {
    DUMPOUT << head;
    if (sizeof...(Tail) > 0) {
        DUMPOUT << ", ";
    }
    dump_func(std::move(tail)...);
}
#ifdef DEBUG
#define DEB
#define dump(...)                                                              \
    DUMPOUT << "  " << string(#__VA_ARGS__) << ": "                            \
            << "[" << to_string(__LINE__) << ":" << __FUNCTION__ << "]"        \
            << endl                                                            \
            << "    ",                                                         \
        dump_func(__VA_ARGS__)
#else
#define DEB if (false)
#define dump(...)
#endif
#endif




signed main() {
    int N, U, V;
    cin >> N >> U >> V;
    U--; V--;
    vector<vector<int>> G(N);
    rep(i,N-1){
        int a, b;
        cin >> a >> b;
        a--; b--;
        G[a].push_back(b);
        G[b].push_back(a);
    }
    // uからの距離
    vector<int> dist_u(N, INF);
    {
        vector<bool> seen(N, false);
        queue<int> q;
        q.push(U);
        seen[U] = true;
        dist_u[U] = 0;

        while (!q.empty()) {
            int u = q.front(); q.pop();
            for (auto v : G[u]) {
                if (seen[v]) continue;
                else {
                    if(dist_u[u]+1 < dist_u[v]){
                        dist_u[v] = dist_u[u] + 1;
                    }
                    q.push(v);
                    seen[v] = true;
                }
            }
        }
    }
    // vからの距離
    vector<int> dist_v(N, INF);
    {
        vector<bool> seen(N, false);
        queue<int> q;
        q.push(V);
        seen[V] = true;
        dist_v[V] = 0;

        while (!q.empty()) {
            int u = q.front(); q.pop();
            for (auto v : G[u]) {
                if (seen[v]) continue;
                else {
                    if(dist_v[u]+1 < dist_v[v]){
                        dist_v[v] = dist_v[u] + 1;
                    }
                    q.push(v);
                    seen[v] = true;
                }
            }
        }
    }

    dump(dist_u);
    dump(dist_v);
    
    int m=-INF;
    rep(i,N){
        if(dist_u[i] < dist_v[i]) m = max(m,dist_v[i]);
    }
    cout << m-1 << endl;
}


Measured memory_bytes: 8064