#include <iostream>
#include <cstdio>
#include <math.h>
#include <algorithm>
#include <bitset>
#include <vector>
using namespace std;
const int maxn=3e5+5;
int n, a, b;
vector < int > ms[maxn];
vector < int > put;
int val[maxn];
bitset < maxn > bio;
bool dfs(int x, int prosl){
put.push_back(x);
if(x==b){
return 1;
}
for(int i=0; i<ms[x].size(); i++){
if(ms[x][i]!=prosl){
if(dfs(ms[x][i], x)){
return 1;
}
}
}
put.pop_back();
return 0;
}
int siri(int x, int prosl){
if(bio[x]){
// cout << "NE" << endl;
return 0;
}
vector < int > svi;
for(int i=0; i<ms[x].size(); i++){
if(ms[x][i]!=prosl && !bio[ms[x][i]]){
svi.push_back(siri(ms[x][i], x));
}
}
if(svi.empty()){
val[x]=1;
return 1;
}
sort(svi.begin(), svi.end());
val[x]=0;
for(int i=0; i<svi.size(); i++){
val[x]=max(val[x], svi[i]+(int)svi.size()-i);
}
// cout << "sirim " << x << " " << val[x] << " " << svi.size() << endl;;
return val[x];
}
int binary1(){
int lo=1, hi=put.size()-1;
int mid;
while(lo<hi){
mid=(lo+hi)/2+1;
bio[mid]=1;
if(siri(put[1], a)>val[a]){
hi=mid-1;
}
else{
lo=mid;
}
bio[mid]=0;
}
return lo;
}
int binary2(){
int lo=0, hi=put.size()-2;
int mid;
while(lo<hi){
mid=(lo+hi)/2;
bio[mid]=1;
if(siri(put[put.size()-2], b)>val[b]){
lo=mid+1;
}
else{
hi=mid;
}
bio[mid]=0;
}
return lo;
}
int ternary(int lo, int hi){
int mid;
int val1, val2;
int val3, val4;
// cout << put[1] << " " << put[put.size()-2] << '\n';
while(lo<hi){
mid=(lo+hi)/2+1;
bio[put[mid]]=1;
// cout << "poc" << endl;
val1=siri(put[1], a);
bio[put[mid]]=0;
bio[put[mid-1]]=1;
// cout << "poc" << endl;
val2=siri(put[put.size()-2], b);
// cout << "poc" << endl;
val3=siri(put[1], a);
bio[put[mid-1]]=0;
bio[put[mid-2]]=1;
// cout << "poc" << endl;
val4=siri(put[put.size()-2], b);
bio[put[mid-2]]=0;
// cout << mid << endl;
// cout << val1 << " " << val2 << " " << val3 << " " << val4 << '\n';
if(max(val1, val2)>max(val3, val4)){
hi=mid-1;
}
else{
lo=mid;
}
}
int sol=min(max(val1, val2), max(val3, val4));
if(lo==1){
sol=max(sol, max(val[a]-1, val[b]));
}
else if(lo==put.size()-1){
sol=max(sol, max(val[a], val[b]-1));
}
else{
sol=max(sol, max(val[a], val[b]));
}
return min(max(val1, val2), max(val3, val4));
}
int main(){
ios_base::sync_with_stdio(false);
cin.tie(0);
cout.tie(0);
cin >> n >> a >> b;
a--;
b--;
int c, d;
for(int i=0; i<n-1; i++){
cin >> c >> d;
c--;
d--;
ms[c].push_back(d);
ms[d].push_back(c);
}
dfs(a, a);
siri(a, put[1]);
siri(b, put[put.size()-2]);
int lo, hi;
lo=binary1();
hi=binary2();
int sol=1e9;
if(hi+1<=lo){
if(hi==0){
sol=min(sol, max(val[a]-1, val[b]));
}
if(lo==put.size()-1){
sol=min(sol, max(val[a], val[b]-1));
}
sol=min(sol, max(val[a], val[b]));
}
else{
sol=ternary(lo, hi+1);
}
cout << sol << '\n';
return 0;
}
