#include <iostream>
#include <iomanip>
#include <climits>
#include <stack>
#include <fstream>
#include <algorithm>
#include <cstdio>
#include <cstring>
#include <cmath>
#include <string>
#include <vector>
#include <deque>
#include <queue>
#include <set>
#include <map>
#include <cassert>
#define FOR(i,n) for(int i=0,_n=n;i<_n;i++)
#define FORR(i,s,n) for(int i=s,_n=n;i<_n;i++)
#define mp make_pair
#define pb push_back
#define pii pair<int,int>
#define pli pair<ll,int>
#define vi vector<int>
#define fs first
#define sec second
#define maxn 1000000
using namespace std;
typedef long long ll;
const ll MOD = 1000000007LL;
vi drevo[maxn+5];
int parent[maxn+5];
int cena[maxn+5];
vector <pii> subtrees;
int ispath[maxn+5];
int root, mis, n;
int dfs1(int poz, int pr){//dobi podatke o starsu, vrne podatek o tem, ali se nahaja na poti do misi oz. kako dalec je
int stars=-1;
FOR(i,drevo[poz].size())if(drevo[poz][i]==pr)stars=i;
if(stars!=-1)drevo[poz].erase(drevo[poz].begin()+stars);
parent[poz]=pr;
int napoti=-1;
if(poz==mis)napoti=0;
FOR(i,drevo[poz].size())napoti=max(napoti,dfs1(drevo[poz][i],poz));
ispath[poz]=napoti;
if(napoti==-1)return napoti;
return napoti+1;
}
void dfs2(int poz, int c){//izracuna ceno sebe in sinov in po potrebi doda v subtrees
if(drevo[poz].size()==0){//list
cena[poz]=c;
return;
}
if(ispath[poz]==-1){
int nyuc=c+drevo[poz].size();
FOR(i,drevo[poz].size())dfs2(drevo[poz][i],nyuc);
if(drevo[poz].size()==1){
cena[poz]=c+1;
return;
}
int best=-1;
int sbest=-1;
FOR(i,drevo[poz].size()){
if(cena[drevo[poz][i]]>best){
sbest=best;
best=cena[drevo[poz][i]];
continue;
}
if(cena[drevo[poz][i]]>sbest)sbest=cena[drevo[poz][i]];
}
cena[poz]=sbest;
return;
}
int nyuc=c+drevo[poz].size()-1;//ne stejemo povezave naprej po poti
if(poz==root)nyuc=0;
if(poz==mis)nyuc++;//ce smo pri misi, potem ni nadaljevanja poti
FOR(i,drevo[poz].size())dfs2(drevo[poz][i],nyuc);
if(poz==root)return;//v korenu nam ni treba nic vec rezat
FOR(i,drevo[poz].size())if(ispath[drevo[poz][i]]==-1)subtrees.pb(mp(ispath[poz],cena[drevo[poz][i]]));
}
int main(){
scanf("%d%d%d",&n,&root,&mis);
root--;
mis--;
FOR(i,n-1){
int a,b;
scanf("%d%d",&a,&b);
a--,b--;
drevo[a].pb(b);
drevo[b].pb(a);
}
int pathl = dfs1(root,-1);
dfs2(root,0);
sort(subtrees.begin(),subtrees.end());
int l=0,d=n;
while(l<d){
int s=(l+d)/2;
int x=0,y=0,pt=0;
FOR(i,pathl){
if(pt>=subtrees.size()){
d=s;
break;
}
x++;
int ydelta=0;
while(pt<subtrees.size() && subtrees[pt].fs<=i){
if(subtrees[pt].sec+y>s){//rezat treba
ydelta++;
x--;
}
pt++;
}
y+=ydelta;
if(x<0 || y>s){//ni bilo dovolj rezov
l=s+1;
break;
}
}
}
printf("%d\n",l);
return 0;
}
