#include <bits/stdc++.h>
#define pb push_back
#define F first
#define S second
using namespace std;
typedef long long ll;
typedef string str;
const ll Mod = 1e9 + 7;
const int Maxn = 1e6 + 100;
vector<ll> G[Maxn];
ll dp[Maxn], son[Maxn], m1[Maxn], m2[Maxn];
ll t, m;
vector<ll> V;
vector<ll> S[Maxn];
bool DFS(ll u, ll p){
bool res = (u == m), r2;
ll cnt = 0;
ll mx = 0;
ll mx2 = 0;
for(auto adj : G[u]){
if(adj == p) continue;
r2 =DFS(adj, u);
res |= r2;
if(!r2){
S[u].pb(adj);
cnt ++;
if(dp[adj] > mx){
mx2 = mx;
mx = dp[adj];
} else mx2 = max(mx2, dp[adj]);
}
}
dp[u] = cnt + mx2;
m2[u] = mx2;
m1[u] = mx;
son[u] = cnt;
if(res){
V.pb(u);
}
return res;
}
ll dp2[Maxn];
vector< pair<ll, ll> > A;
ll C[Maxn];
bool CMP(ll i, ll j){
return dp[i] > dp[j];
}
ll ps[Maxn];
bool check(ll x){
ll M = V.size();
ll c = 0;
for(int i = 0; i < M - 1; i++){
ll SS = S[V[i]].size();
for(int j = 0; j < SS; j++){
if(c + dp[S[V[i]][j]] + (SS - j) + ps[i] > x) {
c++;
} else break;
}
//cerr << c << '\n';
if(c > i + 1) return false;
}
//cerr << "!!!" << x << '\n';
return (c <= x);
}
int main(){
ios::sync_with_stdio(0); cin.tie(0); cout.tie(0);
ll n;
cin >> n >> t >> m;
ll u, v;
for(int i = 1; i < n; i++){
cin >> u >> v;
G[u].pb(v);
G[v].pb(u);
}
DFS(t, -1);
ll sz = V.size();
dp2[sz-1] = 0;
ll sm = 0;
ps[sz-1] = 0;
for(int i = sz - 2; i >= 0; i--){
ps[i] = sm;
sm += son[V[i]];
//cerr << son[V[i]] << '\n';
ll K = 0;
sort(S[V[i]].begin(), S[V[i]].end(), CMP);
for(auto ch : S[V[i]]){
//cerr << '!' << ch << ' ' << dp[ch] << ' ' << sm << '\n';
A.pb({dp[ch] + sm - K, i + 1});
K ++;
}
dp2[i] = min( max(dp2[i + 1], sm-son[V[i]] + m1[V[i]] + son[V[i]]), 1LL + max(dp2[i + 1], sm-son[V[i]] + m2[V[i]] + son[V[i]] - 1) );
}
sort(A.begin(), A.end()); reverse(A.begin(), A.end());
ll M = A.size();
ll cnt = 1;
//cerr << '\n';
//dp2[0] = 100000000;
/*
ll ans = min(dp2[0], (M > 0 ? A[0].F : 0));
bool fl=true;;
A.pb({0, 0});
for(int i = 0; i < M; i++){
for(int j = A[i].S; j <= n; j++){
C[j] ++;
if(C[j] > j) fl = false;
}
//cerr << i << " " << fl << '\n';
if(fl) ans = min(ans, i + 1 + A[i + 1].F);
cerr << i + 1 + A[i + 1].F << '\n';
}
cout << ans << '\n';
*/
//cout << dp2[0] << '\n';
ll l = -1, r = n, mid;
while(l + 1 < r){
mid = (l + r) >> 1;
if(check(mid)) r = mid;
else l = mid;
}
//cerr << check(1) << '\n';
cout << r;
return 0;
}
/*
5 1 2
1 2
2 3
2 4
3 5
10 1 4
1 2
2 3
2 4
3 9
3 5
4 7
4 6
6 8
7 10
7 1 3
1 2
2 3
2 4
4 5
2 6
6 7
*/
