#include<bits/stdc++.h>
using namespace std;
typedef long long ll;
#define F first
#define S second
#define pii pair<int, int>
#define pli pair<ll, int>
#define pil pair<int, ll>
#define pll pair<ll, ll>
#define eb emplace_back
#define all(v) v.begin(), v.end()
#define rep(i, n) for (int i = 0; i < (n); ++i)
#define rep3(i, l, n) for (int i = l; i < (n); ++i)
#define sz(v) (int)v.size()
#define endl '\n'
const int inf = 1000000007;
const ll INF = 1e18;
// int mod = 998244353;
int mod = 1000000007;
#define abs(x) (x >= 0 ? x : -(x))
#define lb(v, x) (int)(lower_bound(all(v), x) - v.begin())
#define ub(v, x) (int)(upper_bound(all(v), x) - v.begin())
template<typename T1, typename T2> inline bool chmin(T1 &a, T2 b) { if (a > b) { a = b; return 1; } return 0; }
template<typename T1, typename T2> inline bool chmax(T1 &a, T2 b) { if (a < b) { a = b; return 1; } return 0; }
ll gcd(ll a, ll b) { if (b == 0) return a; return gcd(b, a % b); }
ll lcm(ll a, ll b) { return a / gcd(a, b) * b; }
template<typename T, typename U> T pow_(T a, U b) { return b ? pow_(a * a, b / 2) * (b % 2 ? a : 1) : 1; }
ll modpow(ll a, ll b, ll _mod) { return b ? modpow(a * a % _mod, b / 2, _mod) * (b % 2 ? a : 1) % _mod : 1; }
template<class T, class U> ostream& operator << (ostream& os, const pair<T, U>& p) { os << p.F << " " << p.S; return os; }
template<class T> ostream& operator << (ostream& os, const vector<T>& vec) { rep(i, sz(vec)) { if (i) os << " "; os << vec[i]; } return os; }
template<typename T> inline istream& operator >> (istream& is, vector<T>& v) { rep(j, sz(v)) is >> v[j]; return is; }
template<class T, class T2> inline void add(T &a, T2 b) { a += b % mod; if (a >= mod) a -= mod; }
template<class T> void operator += (vector<T>& v, vector<T> v2) { rep(i, sz(v2)) v.eb(v2[i]); }

void solve();

int main() {
    ios::sync_with_stdio(false);
    cin.tie(0);
    cout.tie(0);
    cout << fixed << setprecision(12); // debuged
    int T;
    T = 1;
    while (T--) solve();
}

void solve() {
    int n;
    cin >> n;
    if (n == 2) {
        cout << 0 << endl;
        return;
    }
    
    int s, t;
    cin >> s >> t;
    --s; --t;
    vector<vector<pil> > G(n);
    rep(i, n - 1) {
        int a, b;
        cin >> a >> b;
        --a; --b;
        G[a].eb(b, 1);
        G[b].eb(a, 1);
    }
    
    auto dijkstra = [&](int s) {
        vector<ll> d(n, INF);
        priority_queue<pli, vector<pli>, greater<pli> > q;
        
        d[s] = 0;
        q.emplace(0, s);
        
        while (!q.empty()) {
            pli p = q.top();
            q.pop();
            
            int v = p.S;
            if (d[v] < p.F) continue;
            rep(i, sz(G[v])) {
                int nv = G[v][i].F;
                ll cost = G[v][i].S;
                if (chmin(d[nv], d[v] + cost)) {
                    q.emplace(d[nv], nv);
                }
            }
        }
        return d;
    };
    auto d1 = dijkstra(s), d2 = dijkstra(t);
    // cerr << d1 << endl << d2 << endl;
    
    int ma = 1, g = s; // d1[s]<d2[s]
    rep(i, n) if (d1[i] < d2[i] && chmax(ma, d2[i])) g = i;
    
    auto d3 = dijkstra(g);
    rep(i, n) {
        auto calc = [&](int x) {
            int mi = inf, nv = -1;
            for (pil p : G[x]) {
                if (chmin(mi, d3[p.F])) {
                    nv = p.F;
                }
            }
            return nv;
        };
        s = calc(s);
        // cerr << "s t " << s << " " << t << endl;
        if (s == t) {
            cout << i << endl;
            return;
        }
    
        t = calc(t);
        // cerr << "s t " << s << " " << t << endl;
        if (s == t) {
            cout << i + 1 << endl;
            return;
        }
    }
}


Measured memory_bytes: 13856