#include <bits/stdc++.h>
#define range(i, l, r) for(long long int (i) = (l); (i) < (r); (i)++)
#define reversed_range(i, l, r) for (long long int (i) = (r) - 1; (i) >= l; (i)--)
using namespace std;
template <typename T>
using vec = vector<T>;
using lint = long long;
using ulint = unsigned long long;
using pint = pair<int, int>;
using plint = pair<lint, lint>;

template <typename S, typename T>
ostream& operator <<(ostream& os, pair<S, T> p) {
    os << "(";
    os << p.first << ", " << p.second;
    return os << ")";
}

template <typename T>
ostream& operator <<(ostream& os, vec<T> v) {
    os << "[";
    if (v.size() == 0) return os << "]";
    for (int i = 0; i < v.size() - 1; i++) {
        os << v.at(i) << ", ";
    }
    return os << v.at(v.size() - 1) << "]";
}

template <typename T>
ostream& operator <<(ostream& os, set<T>& s) {
    os << "{";
    if (s.begin() == s.end()) return os << "}";
    auto it_first_item = s.begin();
    cout << *it_first_item;
    for (auto it = ++it_first_item; it != s.end(); it++) {
        cout << ", " << *it;
    }
    return cout << "}";
}

template <typename T>
ostream& operator <<(ostream& os, unordered_set<T>& s) {
    os << "{";
    if (s.begin() == s.end()) return os << "}";
    auto it_first_item = s.begin();
    cout << *it_first_item;
    for (auto it = ++it_first_item; it != s.end(); it++) {
        cout << ", " << *it;
    }
    return cout << "}";
}

template <typename K, typename V>
ostream& operator <<(ostream& os, map<K, V> m) {
    os << "{";
    if (m.begin() == m.end()) return os << "}";
    auto it_first_item = m.begin();
    cout << it_first_item->first << ": " << it_first_item->second;
    for (auto it = ++it_first_item; it != m.end(); it++) {
        cout << ", " << it->first << ": " << it->second;
    }
    return os << "}";
}

template <typename K, typename V>
ostream& operator <<(ostream& os, unordered_map<K, V> m) {
    os << "{";
    if (m.begin() == m.end()) return os << "}";
    auto it_first_item = m.begin();
    cout << it_first_item->first << ": " << it_first_item->second;
    for (auto it = ++it_first_item; it != m.end(); it++) {
        cout << ", " << it->first << ": " << it->second;
    }
    return os << "}";
}

lint pow(lint num, lint e, lint MOD) {
    lint res = 1;
    lint cur_num = num;
    while (e) {
        if (e & 1) {
            res *= cur_num;
            res %= MOD;
        }
        cur_num *= cur_num;
        cur_num %= MOD;
        e >>= 1;
    }
    return res;
}

int main() {
    cin.tie(0); cout.tie(0);
    ios::sync_with_stdio(false);
    cout << fixed << setprecision(10);
    lint N, u, v; cin >> N >> u >> v;
    u--; v--;
    vec<vec<lint>> adj(N);
    range(i, 0, N - 1) {
        lint a, b; cin >> a >> b;
        a--; b--;
        adj.at(a).emplace_back(b);
        adj.at(b).emplace_back(a);
    }
    auto calculate_distance_from = [&](const lint s) {
        vec<lint> distance(N, -1);
        distance.at(s) = 0;
        deque<lint> nodes_in_process;
        nodes_in_process.emplace_back(s);
        while (!nodes_in_process.empty()) {
            lint node = nodes_in_process.front(); nodes_in_process.pop_front();
            for (const lint& neighbor : adj.at(node)) {
                if (distance.at(neighbor) != -1) continue;
                distance.at(neighbor) = distance.at(node) + 1;
                nodes_in_process.emplace_back(neighbor);
            }
        }
        return distance;
    };
    vec<lint> distance_from_takahashi = calculate_distance_from(u);
    vec<lint> distance_from_aoki = calculate_distance_from(v);
    lint max_dist = 1;
    range(node, 0, N) {
        if (distance_from_takahashi.at(node) < distance_from_aoki.at(node)) {
            max_dist = max(max_dist, distance_from_aoki.at(node));
        }
    }
    cout << max_dist - 1 << "\n";
}

Measured memory_bytes: 8064