#include <iostream>
#include <iomanip>
#include <cmath>
#include <functional>
#include <algorithm>
#include <numeric>
#include <vector>
#include <list>
#include <queue>
#include <stack>
#include <deque>
#include <map>
#include <set>
#include <unordered_map>
#include <unordered_set>
#include <boost/range/irange.hpp>
#include <boost/container/static_vector.hpp>
#include <boost/utility/string_ref.hpp>
#include <boost/multiprecision/cpp_int.hpp>
#include <boost/math/common_factor_rt.hpp>

class input
{
    std::istream& cin;

public:
    input() : cin(std::cin)
    {
        std::cin.tie(nullptr);
        std::ios::sync_with_stdio(false);
    }

    template <class T>
    inline input& operator>>(T& var)
    {
        cin >> var;
        return *this;
    }
    template <class T>
    inline input& operator,(T& var)
    {
        cin >> var;
        return *this;
    }

    template <class T>
    inline T read()
    {
        T var;
        cin >> var;
        return var;
    }

    template <class InputIterator>
    inline void read(InputIterator head, std::size_t n)
    {
        for (std::size_t i = 0; i < n; ++i) {
            cin >> *head;
            ++head;
        }
    }

    template <class T>
    inline std::vector<T> read(std::size_t n)
    {
        std::vector<T> res;
        res.reserve(n);

        for (std::size_t i = 0; i < n; ++i) {
            T tmp;
            cin >> tmp;
            res.emplace_back(tmp);
        }
        return res;
    }

    template <class T, class F>
    inline void online(std::size_t n, F&& f)
    {
        T var;
        for (std::size_t i = 0; i < n; ++i) {
            cin >> var;
            f(var);
        }
    }
};
template <class T, std::size_t n>
std::istream& operator>>(std::istream& in, std::array<T, n>& var)
{
    for (std::size_t i = 0; i < n; ++i) {
        in >> var[i];
    }
    return in;
}

class output
{
    std::ostream& cout;

public:
    template <bool nl>
    class proxy
    {
        std::ostream& cout;

    public:
        proxy() : cout(std::cout) {}
        ~proxy() { final(); }
        template <class T>
        proxy& operator,(T&& val)
        {
            cout << ' ' << val;
            return *this;
        }
        template <bool f = nl>
        auto final() -> std::enable_if_t<f> { cout << '\n'; }
        template <bool f = nl>
        auto final() -> std::enable_if_t<!f> {}
    };

public:
    output() : cout(std::cout)
    {
        cout << std::fixed << std::setprecision(15);
    }

    template <class T>
    proxy<true> operator,(T&& val)
    {
        cout << val;
        return {};
    }
    proxy<false> noreturn() { return {}; }

    template <class T>
    void print(const T& container)
    {
        bool first = true;
        for (auto&& x : container) {
            if (!first) {
                cout << ' ';
            }
            cout << x;
            first = false;
        }
        cout << '\n';
    }
};

void turn(std::vector<int>& tree, int root)
{
    if (root != tree[0]) {
        turn(tree, tree[root]);
    }
    int prevRoot = tree[0];
    tree[0] = tree[prevRoot] = root;
    tree[root] = -1;
}

void depth(const std::vector<int>& tree, std::vector<int>& d, int node)
{
    if (d[node] != -1) return;
    if (node == tree[0]) {
        d[node] = 0;
        return;
    }
    if (d[tree[node]] == -1) {
        depth(tree, d, tree[node]);
    }
    d[node] = d[tree[node]] + 1;
}

void prog()
{
    input in;
    output out;
    int n, u, v;
    in >> n, u, v;
    std::vector<int> tree(n + 1, -1);
    tree[0] = 1;
    for (int i : boost::irange(0, n - 1)) {
        int a, b;
        in >> a, b;
        if (a > b) std::swap(a, b);
        tree[b] = a;
    }
    std::vector<int> ud(n + 1, -1), vd(n + 1, -1);
    turn(tree, u);
    if (std::count(std::begin(tree), std::end(tree), u) == 1) {
        out, 0;
        return;
    }
    for (int i : boost::irange(1, n + 1)) {
        depth(tree, ud, i);
    }
    turn(tree, v);
    for (int i : boost::irange(1, n + 1)) {
        depth(tree, vd, i);
    }
    int res = -1;
    for (int i : boost::irange(1, n + 1)) {
        if (ud[i] < vd[i]) {
            res = std::max(res, vd[i]);
        }
    }
    out, res - 1;
}

int main()
{
    prog();
    return 0;
}
