#include "bits/stdc++.h"
// Begin Header {{{
#define all(x)          (x).begin(), (x).end()
#define rep(i, n)       for (i64 i = 0, i##_limit = (n); i < i##_limit; ++i)
#define reps(i, s, t)   for (i64 i = (s), i##_limit = (t); i <= i##_limit; ++i)
#define repr(i, s, t)   for (i64 i = (s), i##_limit = (t); i >= i##_limit; --i)
#define var(Type, ...)  Type __VA_ARGS__; input(__VA_ARGS__)
#ifndef DBG
#define trace(...)
#endif
using namespace std;
using i64 = int_fast64_t;
using pii = pair<i64, i64>;
template <class T, class U> inline bool chmax(T &a, const U &b) { return b > a && (a = b, true); }
template <class T, class U> inline bool chmin(T &a, const U &b) { return b < a && (a = b, true); }
inline i64  sigma(i64 n)            { return (n * (n + 1) >> 1); }
inline i64  updiv(i64 a, i64 b)     { return (a + b - 1) / b; }
inline i64  sqr(i64 n)              { return n * n; }
inline string to_string(char c)     { return string(1, c); }
constexpr int INF  = 0x3f3f3f3f;
constexpr i64 LINF = 0x3f3f3f3f3f3f3f3fLL;

template <class T>
inline vector<T> make_v(const T &initValue, size_t sz) {
    return vector<T>(sz, initValue);
}
template <class T, class... Args>
inline auto make_v(const T &initValue, size_t sz, Args... args) {
    return vector<decltype(make_v<T>(initValue, args...))>(sz, make_v<T>(initValue, args...));
}

inline void input() {}
template <class Head, class... Tail>
inline void input(Head &head, Tail&... tail) { cin >> head; input(tail...); }

inline void print() { cout << "\n"; }
template <class Head, class... Tail>
inline void print(Head &&head, Tail&&... tail) {
    cout << head;
    if (sizeof...(tail)) cout << ' ';
    print(forward<Tail>(tail)...);
}

template <class T>
inline ostream& operator<< (ostream &out, const vector<T> &vec) {
    static constexpr const char *delim[] = { " ", "" };
    for (const auto &e : vec) out << e << delim[&e == &vec.back()];
    return out;
}

template <typename Func>
struct FixPoint : Func {
    inline constexpr FixPoint(Func &&f) noexcept : Func(forward<Func>(f)) {}

    template <typename... Args>
    inline decltype(auto) operator()(Args &&... args) const {
        return Func::operator()(*this, forward<Args>(args)...);
    }
};

template< typename Func >
inline decltype(auto) makeFixPoint(Func &&f) {
    return FixPoint< Func >{forward< Func >(f)};
}
// }}} End Header

// graph {{{
struct Edge { // {{{
    int src, to;
    int64_t cost = 0;

    inline Edge() noexcept {}
    inline Edge(int s,  int t, int64_t c = 1) noexcept : src(s), to(t), cost(c) {}

    inline bool operator<(const Edge &rhs) const noexcept { return cost < rhs.cost; }
    inline bool operator>(const Edge &rhs) const noexcept { return rhs < *this; }

    inline operator int() const noexcept { return to; }

    inline Edge& operator=(int rhs_to) noexcept { this->to = rhs_to; return *this; }
};
// }}}

class Graph : public vector<vector<Edge>> { // {{{
    using super = vector<vector<Edge>>;
public:
    Graph() noexcept {}
    Graph(int V) noexcept : super(V) {}

    Graph& addEdge(int from, int to, int64_t cost = 1) noexcept {
        super::operator[](from).emplace_back(from, to, cost);
        return *this;
    }

    Graph& addEdge(const Edge &e) noexcept {
        return addEdge(e.src, e.to, e.cost);
    }

    friend ostream& operator<<(ostream &out, const Graph &g) {
        for (int i = 0; i < g.size(); ++i) {
            out << '[' << setw(2) << i << ']';
            for (const auto &e : g[i]) {
                out << ' ' << setw(2) << e.to;
            }
            out << '\n';
        }
        return out;
    }
}; // }}}
// }}}

class DoublingLCA { // {{{
public:
    const int V;
    const int LOG_V;
    Graph G;
    vector<int> depth;
    vector<int64_t> weightSum;
    vector<vector<int>> table;  // table[k][v] = ノード v から根へ 2^k だけ登った先のノード番号

    DoublingLCA() = delete;

    // ノード番号 [0, n) を管理する
    DoublingLCA(int n) :
        V(n),
        LOG_V(32 - __builtin_clz(V)),   // ceil(log2(V))
        G(V),
        depth(V),
        weightSum(V),
        table(LOG_V, vector<int>(V, -1)) {}


    // u-v間に無向辺を張る。重みは省略可
    inline void addEdge(int u, int v, int64_t weight = 0) noexcept {
        G.addEdge(u, v, weight);
        G.addEdge(v, u, weight);
    }

    // 前計算
    void build(int root) noexcept {
        dfs(root, -1, 0, 0);
        for (int k = 0; (k + 1) < LOG_V; ++k) {
            for (int v = 0; v < V; ++v) {
                if (table[k][v] == -1) continue;
                table[k + 1][v] = table[k][table[k][v]];
            }
        }
    }

    // u,v のLCAを求める
    inline int lca(int u, int v) const noexcept {
        if (depth[u] > depth[v]) swap(u, v);
        for (int i = LOG_V - 1; i >= 0; --i) {
            if ((depth[v] - depth[u]) & (1 << i)) v = table[i][v];
        }

        if (u == v) return u;

        for (int i = LOG_V - 1; i >= 0; --i) {
            if (table[i][u] != table[i][v]) {
                u = table[i][u];
                v = table[i][v];
            }
        }
        return table[0][u];
    }

    // v から根へ k だけ登った先のノードを求める
    inline int climb(int v, int k) const noexcept {
        int i = 0;
        while(k > 0) {
            if (k & 1) v = table[i][v];
            ++i;
            k >>= 1;
        }
        return v;
    }

    // u-v間のパスの重みの和を求める
    inline int pathWeight(int u, int v) const noexcept {
        return weightSum[u] + weightSum[v] - 2 * weightSum[lca(u, v)];
    }

    // u-v間のパスの「辺の本数」を求める
    inline int pathLength(int u, int v) const noexcept {
        return depth[u] + depth[v] - 2 * depth[lca(u, v)];
    }

private:
    void dfs(int v, int par, int dep, int64_t ws) noexcept {
        depth[v] = dep;
        weightSum[v] = ws;
        table[0][v] = par;
        for (const auto &e : G[v]) {
            if (e.to != par) dfs(e.to, v, dep + 1, ws + e.cost);
        }
    }
};  // }}}

signed main()
{
    ios::sync_with_stdio(false); cin.tie(nullptr);

    var(int, N, T, A);
    vector<int> deg(N);

    DoublingLCA G(N+1);
    rep(i, N-1) {
        var(int, a, b);
        G.addEdge(a, b);
        ++deg[a];
        ++deg[b];
    }

    G.build(1);
    int ans = 0;

    reps(i, 1, N) {
        if (deg[i] != 1) continue;
        if (G.pathLength(T, i) >= G.pathLength(A, i)) continue;
        chmax(ans, G.pathLength(A, i) - 1);
    }

    print(ans);

    return 0;
}
