WEDDING - ĐT

// Make CSP great again
#include <bits/stdc++.h>
#define TASK "WEDDING"
using namespace std;
const int N = 3e4;
bool Visited[N + 1];
int d[N + 1], n, a, b;
vector<int> Adj[N + 1];
void read()
{
   cin >> n;
   for (int i = 1; i < n; ++ i)
   {
       int u, v;
       cin >> u >> v;
       Adj[u].push_back(v);
       Adj[v].push_back(u);
       Visited[i] = false;
   }
   Visited[n] = false;
   cin >> a >> b;
   Visited[a] = true;
   Visited[b] = true;
   d[a] = d[b] = 1;
}
bool DFS2(int u, int k)
{
    if (d[u] == k)
    {
        return 1;
    }
    bool ans = false;
    for (int v : Adj[u])
    {
        if (!Visited[v])
        {
            d[v] = d[u] + 1;
            Visited[v] = true;
            ans |= DFS2(v, k);
            Visited[v] = false;
        }
    }
    return ans;
}
bool DFS1(int u, int k)
{
    bool ans = false;
    if (d[u] == k)
    {
        ans |= DFS2(b, k);
        return ans;
    }
    for (int v : Adj[u])
    {
        if (!Visited[v])
        {
            d[v] = d[u] + 1;
            Visited[v] = true;
            ans |= DFS1(v, k);
            Visited[v] = false;
        }
    }
    return ans;
}
void solve()
{
    int low = 1, high = n;
    while (low <= high)
    {
        int mid = (low + high)/2;
        if (DFS1(a, mid))
        {
            low = mid + 1;
        }
        else
        {
            high = mid - 1;
        }
    }
    cout << high;
}
int main()
{
    ios_base::sync_with_stdio(false);
    cin.tie(nullptr);
    freopen(TASK".INP", "r", stdin);
    freopen(TASK".OUT", "w", stdout);
    int t = 1;
    bool typetest = false;
    if (typetest)
    {
        cin >> t;
    }
    for (int __ = 1; __ <= t; ++ __)
    {
        read();
        solve();
    }
}