#include <bits/stdc++.h>
#define ll long long
#define pb push_back
#define eb emplace_back
#define pu push
#define ins insert
#define fi first
#define se second
#define all(a) a.begin(),a.end()
#define bruh ios_base::sync_with_stdio(false);cin.tie(0);cout.tie(0);
#define fu(x,a,b) for (auto x=a;x<=b;x++)
#define fd(x,a,b) for (auto x=a;x>=b;x--)
#define int ll
using namespace std;
#pragma GCC optimize("Ofast")
#pragma GCC optimize("unroll-loops")
//mt19937 mt(chrono::steady_clock::now().time_since_epoch().count());
/*
Competitive Programming notes that I need to study & fix my dumbass self:
1. Coding:
- Always be sure to check the memory of arrays (maybe use vectors), for loops, I don't know
- Always try to maximize the memory if possible, even if you are going for subtasks
- Do not exploit #define int long long, it will kill you
2. Stress:
- Don't be cocky and think stressing with your dumbass brute-force solution will give you guaranteed AC.
- Always try generating big testcases and try if they run
3. Time management:
- Don't overcommit or undercommit, always spend a certain amount of time to think a problem, don't just look at it and say I'm fucked
- Do not spend too much time coding brute-force solutions, they should be easily-codable solutions that don't take up too much time
I hate offline because I am dumb
*/
typedef pair<int, int> ii;
const int N = 1e6+5;
const int mod = 1e9+7;
const int inf = 1e18;
using cd = complex<double>;
const long double PI = acos(-1);
int power(int a,int b) {ll x = 1;if (a >= mod) a%=mod; while (b) {if (b & 1) x = x*a % mod;a = a*a % mod;b>>=1;}return x;}
int n,t,m;
vector<int> adj[N];
int dp[N], par[N];
void dfs(int u, int p)
{
int mx = 0, sec = 0;
for (auto v : adj[u])
{
if (v == p) continue;
par[v] = u;
dfs(v,u);
if (dp[v] >= mx) sec = mx, mx = dp[v];
else if (dp[v] > sec) sec = dp[v];
}
dp[u] = sec + adj[u].size() - 1;
}
int sum = 0;
bool check(int x)
{
int u = m, rem = 0, child = -1, cnt = 0, tot = 0;
while (u != t)
{
rem++;
int add = 0;
for (auto v : adj[u])
{
if (v == child || v == par[u]) continue;
if ((dp[v] + cnt) > (x + tot))
{
if (!rem) return 0;
else rem--, add++;
}
}
cnt += add;
tot += adj[u].size() - (child != -1);
if (par[u] != 0) tot--;
child = u, u = par[u];
}
sum = tot;
return 1;
}
void solve()
{
cin>>n>>t>>m;
for (int i = 1; i < n; i++)
{
int u,v; cin>>u>>v;
adj[u].pb(v), adj[v].pb(u);
}
dfs(t,0);
int l = -1, r = n;
while (l < r-1)
{
int mid = l+r>>1;
if (check(mid)) r = mid;
else l = mid;
}
cout<<r + sum;
}
/*
Go through the mistakes you usually make and revise your code, for god's sake...
*/
signed main()
{
bruh
//freopen("input.inp","r",stdin);
//freopen("output.inp","w",stdout);
solve();
}
