#include <bits/stdc++.h>

using namespace std;

const int N = 1e6 + 10, M = 2e6 + 10;
int n, root, mouse, h[N], e[M], ne[M], idx;
inline void add(int a, int b) {e[idx] = b, ne[idx] = h[a], h[a] = idx ++; }

int fa[N], dp[N], siz[N], sum[N];
void dfs(int u, int f) {
    fa[u] = f; pair<int, int> res = {0, 0};
    for(int i = h[u]; i != -1; i = ne[i]) {
        int v = e[i]; if(v == f) continue;
        siz[u] ++;
    }
    if(u != root) sum[u] = sum[f] + siz[u] - 1 + (u == mouse);
    for(int i = h[u]; i != -1; i = ne[i]) {
        int v = e[i]; if(v == f) continue;
        dfs(v, u);
        if(dp[v] > res.first) res.second = res.first, res.first = dp[v];
        else if(dp[v] > res.second) res.second = dp[v];
    }
    dp[u] = res.second + siz[u];
}

inline bool check(int mid) {
    static bool st[N];
    int lst = 1;
    for(int i = mouse; i != root; i = fa[i]) {
        int Need = 0; st[i] = true;
        for(int j = h[i]; j != -1; j = ne[j]) {
            int v = e[j]; if(v == fa[i] || st[v]) continue;
            if(dp[v] + sum[i] <= mid) continue;
            lst --, Need ++; if(lst < 0) return false;
        }
        mid -= Need, lst ++;
    }
    return mid >= 0;
}

int main() {
    ios::sync_with_stdio(false), cin.tie(0);
    memset(h, -1, sizeof h);

    cin >> n >> root >> mouse;
    for(int i = 1; i < n; i ++) {
        int a, b; cin >> a >> b;
        add(a, b), add(b, a);
    }
    dfs(root, 0);

    int l = 0, r = 2 * n;
    while(l < r) {
        int mid = (l + r) >> 1;
        if(check(mid)) r = mid;
        else l = mid + 1;
    }
    cout << l << '\n';

    return 0;
}