// >>> TEMPLATES
#include <bits/stdc++.h>
using namespace std;
using ll = long long;
using ld = long double;
#define int ll
#define double ld
#define rep(i,n) for (int i = 0; i < (int)(n); i++)
#define rep1(i,n) for (int i = 1; i <= (int)(n); i++)
#define repR(i,n) for (int i = (int)(n)-1; i >= 0; i--)
#define rep1R(i,n) for (int i = (int)(n); i >= 1; i--)
#define loop(i,a,B) for (int i = a; i B; i++)
#define loopR(i,a,B) for (int i = a; i B; i--)
#define all(x) (x).begin(), (x).end()
#define allR(x) (x).rbegin(), (x).rend()
#define pb push_back
#define eb emplace_back
#define mp make_pair
#define fst first
#define snd second
#define INF (numeric_limits<int>::max()/2-1)
#ifdef LOCAL
#define dump(...) cerr << "[" << __LINE__ << ":" << __FUNCTION__ << "] ", dump_impl(#__VA_ARGS__, __VA_ARGS__)
#define say(x) cerr << "[" << __LINE__ << ":" << __FUNCTION__ << "] " << x << endl;
#define debug if (1)
void dump_impl(const char*) { cerr << endl; }
template <class T, class... U> void dump_impl(const char *s, T const& x, U const& ...y) { const char *o = "({[", *e = "]})"; for (int i = 0; *s != '\0'; cerr << *s++) { if (count(o,o+3,*s)) i++; if (count(e,e+3,*s)) i--; if (!i && *s == ',') break; } cerr << " = " << x; if (*s == ',') cerr << ", ", s++; dump_impl(s, y...); }
#else
#define dump(...)
#define say(x)
#define debug if (0)
#endif
template <class T> using pque_max = priority_queue<T>;
template <class T> using pque_min = priority_queue<T, vector<T>, greater<T> >;
template <class T, class = typename T::iterator, class = typename enable_if<!is_same<T, string>::value>::type>
ostream& operator<<(ostream& os, T const& v) { os << "{"; for (auto const& x : v) os << " " << x; return os << " }"; }
template <class T, class = typename T::iterator, class = typename enable_if<!is_same<T, string>::value>::type>
istream& operator>>(istream& is, T &v) { for (auto& x : v) is >> x; return is; }
template <class T, class S> ostream& operator<<(ostream& os, pair<T,S> const& p) { return os << "(" << p.first << ", " << p.second << ")"; }
template <class T, class S> istream& operator>>(istream& is, pair<T,S>& p) { return is >> p.first >> p.second; }
template <size_t i, class T> typename enable_if<i >= tuple_size<T>::value>::type output_tuple(ostream&, T const&) { }
template <size_t i = 0, class T> typename enable_if<i < tuple_size<T>::value>::type
output_tuple(ostream& os, T const& t) { os << (i ? " " : "") << get<i>(t); output_tuple<i+1,T>(os,t); }
template <class... T> ostream& operator<<(ostream& os, tuple<T...> const& t) { return output_tuple(os,t), os; }
struct IOSetup { IOSetup() { cin.tie(nullptr); ios::sync_with_stdio(false); cout << fixed << setprecision(15); } } iosetup;
template <class F> struct FixPoint : private F {
    constexpr FixPoint(F&& f) : F(forward<F>(f)) {}
    template <class... T> constexpr auto operator()(T&&... x) const { return F::operator()(*this, forward<T>(x)...); }
};
struct MakeFixPoint {
    template <class F> constexpr auto operator|(F&& f) const { return FixPoint<F>(forward<F>(f)); }
};
#define MFP MakeFixPoint()|
#define def(name, ...) auto name = MFP [&](auto &&name, __VA_ARGS__)
template <class T, size_t d> struct vec_impl {
    using type = vector<typename vec_impl<T,d-1>::type>;
    template <class... U> static type make_v(size_t n, U&&... x) { return type(n, vec_impl<T,d-1>::make_v(forward<U>(x)...)); }
};
template <class T> struct vec_impl<T,0> { using type = T; static type make_v(T const& x = {}) { return x; } };
template <class T, size_t d = 1> using vec = typename vec_impl<T,d>::type;
template <class T, size_t d = 1, class... Args> auto make_v(Args&&... args) { return vec_impl<T,d>::make_v(forward<Args>(args)...); }
template <class T> void quit(T const& x) { cout << x << endl; exit(0); }
template <class T> constexpr bool chmin(T& x, T const& y) { if (x > y) { x = y; return true; } return false; }
template <class T> constexpr bool chmax(T& x, T const& y) { if (x < y) { x = y; return true; } return false; }
template <class It> constexpr auto sumof(It b, It e) { return accumulate(b,e,typename iterator_traits<It>::value_type{}); }
template <class T> int sz(T const& x) { return x.size(); }
// <<<
// >>> LCA
constexpr int dst(int v) { return v; }
template <class E, class = decltype(E().to)>
constexpr int dst(E const& e) { return e.to; }

template <class E> struct LCA {
    vector<vector<E> > const* g;
    int32_t n,lg,root;
    vector<int32_t> d;
    vector<vector<int32_t> > anc;
    LCA() {}
    LCA(vector<vector<E> > const* g, int root = 0)
        : g(g), n(g->size()), root(root), d(n) {
        lg = 1; while ((1<<lg) < n) lg++; // lg == 1 when n <= 1
        anc.assign(lg, vector<int32_t>(n,-1));
        dfs(root);
        rep (i,lg-1) rep (j,n) if (anc[i][j] >= 0) {
            anc[i+1][j] = anc[i][anc[i][j]];
        }
    }
    void dfs(int x, int p = -1) {
        anc[0][x] = p;
        d[x] = (p == -1 ? 0 : d[p]+1);
        for (auto const& e : (*g)[x]) if (dst(e) != p) dfs(dst(e),x);
    }
    int dep(int a) const { return d[a]; }
    int operator()(int a, int b) const { return lca(a,b); }
    int lca(int a, int b) const {
        if (d[a] > d[b]) swap(a,b);
        for (int dd = d[b]-d[a], i = 0; dd; dd >>= 1, i++) {
            if (dd&1) b = anc[i][b];
        }
        if (a == b) return a;
        for (int i = lg-1; i >= 0; i--) {
            if (anc[i][a] != anc[i][b]) a = anc[i][a], b = anc[i][b];
        }
        return anc[0][a];
    }
    int dist(int a, int b) const { return d[a] + d[b] - 2*d[lca(a,b)]; }
    int climb(int v, int dist) const {
        for (int i = 0; dist && v >= 0; dist >>= 1, i++) {
            if (dist&1) v = anc[i][v];
        }
        return v;
    }
};
template <class E>
auto get_lca(vector<vector<E> > const& g, int root = 0) { return LCA<E>(&g,root); }
// <<<

int32_t main() {
    int n,T,A; cin >> n >> T >> A; T--,A--;
    vector<vector<int> > g(n);
    rep (i,n-1) {
        int a,b; cin >> a >> b; a--,b--;
        g[a].eb(b);
        g[b].eb(a);
    }
    auto lca = get_lca(g,A);
    int d = lca.dist(A,T);
    if (lca.dist(A,T) == 1) quit(0);

    vector<int> dp(n);
    def (dfs, int x, int p) -> int {
        dp[x] = lca.dep(x);
        for (int y : g[x]) {
            if (y == p) continue;
            chmax(dp[x], dfs(y,x));
        }
        return dp[x];
    };
    dfs(A,-1);
    dump(dp);

    int ma = 0;
    int x = T;
    rep (i,d/2+1) {
        chmax(ma, dp[x]-1);
        x = lca.climb(x,+1);
    }

    cout << ma << endl;



}


Measured memory_bytes: 17016