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

#define CPP_STR(x) CPP_STR_I(x)
#define CPP_CAT(x,y) CPP_CAT_I(x,y)
#define CPP_STR_I(args...) #args
#define CPP_CAT_I(x,y) x ## y

using i8  = int8_t;
using u8  = uint8_t;
using i16 = int16_t;
using u16 = uint16_t;
using i32 = int32_t;
using u32 = uint32_t;
using i64 = int64_t;
using u64 = uint64_t;

using f32 = float;
using f64 = double;

#define FOR(i, start, end) for(i64 i = (start), CPP_CAT(i,xxxx_end)=(end); i < CPP_CAT(i,xxxx_end); ++i)
#define REP(i, n) FOR(i, 0, n)
#define REPS(i, n) for(i64 i = 1; i <= n; ++i)
#define RREP(i, n) for(i64 i = n - 1; i >= 0 ; --i)
#define RREPS(i, n) for(i64 i = n; i >= 1; --i)

#define ALL(f,c,...) (([&](decltype((c)) cccc) { return (f)(std::begin(cccc), std::end(cccc), ## __VA_ARGS__); })(c))

#define UNIQUE(v) v.erase( unique(v.begin(), v.end()), v.end() );

#define pb push_back
#define mp make_pair

template<class T>bool chmax(T &a, const T &b) { if (a<b) { a=b; return 1; } return 0; }
template<class T>bool chmin(T &a, const T &b) { if (b<a) { a=b; return 1; } return 0; }

template<typename C>
i64 SIZE(const C& c) { return static_cast<i64>(c.size()); }

template<typename T, size_t N>
i64 SIZE(const T (&)[N]) { return static_cast<i64>(N); }


struct ProconInit {
    static constexpr int IOS_PREC = 15;
    static constexpr bool AUTOFLUSH = false;

    ProconInit() {
        cin.tie(nullptr);
        ios::sync_with_stdio(false);
        cout << fixed << setprecision(IOS_PREC);
        if(AUTOFLUSH)
            cout << unitbuf;
    }
} PROCON_INIT;

vector<i64> G[100'000];

int main() {
    i64 N, s, t; cin >> N >> s >> t;
    s--, t--;
    if (s == t) {
        cout << 1 << endl;
        return 0;
    }
    REP(i, N - 1) {
        i64 a, b; cin >> a >> b;
        a--, b--;
        G[a].pb(b);
        G[b].pb(a);
    }

    auto calc = [&](i64 s) {
        vector<i64> d(N, -1);
        d[s] = 0;
        queue<i64> que;
        que.push(s);
        while (!que.empty()) {
            i64 u = que.front(); que.pop();

            for (i64 v : G[u]) {
                if (d[v] == -1) {
                    d[v] = d[u] + 1;
                    que.push(v);
                }
            }
        }
        return d;
    };

    vector<i64> ds = calc(s);
    vector<i64> dt = calc(t);

    i64 root = t;
    i64 maxd = 0;
    i64 maxv = s;
    REP(i, N) {
        if (i != root && G[i].size() == 1 && ds[i] < dt[i]) {
            if (chmax(maxd, ds[i])) {
                maxv = i;
            }
        }
    }

    auto parent = [&](i64 u) {
        if (u == root) return root;
        i64 p = -1;
        for (i64 v : G[u]) {
            if (dt[v] + 1 == dt[u]) {
                p = v;
                break;
            }
        }
        return p;
    };

    i64 v = parent(parent(maxv));
    cout << (dt[v] + (v != parent(maxv))) << endl;

    return 0;
}