///services.msc / информация... / автоматически
#include<bits/stdc++.h>

#define pb push_back
#define ll long long
#define ld long double
#define F first
#define S second

using namespace std;

mt19937 gen(chrono::system_clock::now().time_since_epoch().count());

const ll M = 1e9 + 7;
const ll N = 2e6 + 7;

int n, t, m, x, y, f[N], d[N], p[N];
vector<int> g[N], a, F[N];
bool fl[N];

void bfs(int v){
    queue<int> q;
    q.push(v);
    p[v] = v;
    while (!q.empty()){
        int u = q.front();
        q.pop();
        for (auto to : g[u])
            if (!p[to]){
                p[to] = u;
                q.push(to);
            }
    }
    return;
}

void dfs(int v, int p){
    if (g[v].size() == 1){
        f[v] = 0;
        return;
    }
    vector<int> all;
    for (auto to : g[v]){
        if (to == p) continue;
        dfs(to, v);
        all.pb(f[to]);
    }
    sort(all.rbegin(), all.rend());
    if (all.size() == 1) f[v] = 1;
    else f[v] = all[1] + all.size();
    return;
}

bool can(int x){
    int cur = 1, y = x;
    for (int i = a.size() - 1; i >= 0; i--){
        int p = 0;
        while (p < F[i].size() && F[i][p] > y) p++;
        cur -= p;
        if (cur < 0) return 0;
        cur++;
        y -= p;
        if (y < 0) return 0;
    }
    return 1;
}

int32_t main(){
    ios_base::sync_with_stdio(0);
#ifdef LOCAL
    freopen("input.txt", "r", stdin);
    freopen("output.txt", "w", stdout);
#endif
    cin >> n >> t >> m;
    for (int i = 1; i < n; i++){
        cin >> x >> y;
        g[x].pb(y);
        g[y].pb(x);
    }
    bfs(t);
    int v = m;
    fl[v] = 1;
    while (v != t){
        a.pb(v);
        v = p[v];
        fl[v] = 1;
    }
    reverse(a.begin(), a.end());
    for (int i = 0; i < a.size(); i++){
        d[i] = (i ? d[i - 1] : 0);
        for (auto to : g[a[i]])
            if (!fl[to]) dfs(to, a[i]), d[i]++;
        for (auto to : g[a[i]])
            if (!fl[to]){
                f[to] += d[i];
                F[i].pb(f[to]);
            }
        sort(F[i].rbegin(), F[i].rend());
    }
    int l = 0, r = 1e9, ans = 1e9;
    while (l <= r){
        int mid = (l + r)/2;
        if (can(mid)){
            ans = min(ans, mid);
            r = mid - 1;
        }
        else l = mid + 1;
    }
    cout << ans;
    return 0;
}