#include<bits/stdc++.h>
typedef long long ll;
#define pb push_back
#define fr first
#define sc second
#define mid ((left+right)>>1)
#define endl '\n'
using namespace std;
int n,t,r,m;
int par[1000023],spec[1000023];
int dp[1000023],ek[1000023];
vector<int>komsu[1000023];
vector<int>res[1000023];
vector<int>specs;
int locs[1000023];
void dfs1(int pos){
ek[pos]=ek[par[pos]]+max(0,int(komsu[pos].size())-2+(pos==r));
if(pos==t)ek[pos]=0;
for(int x:komsu[pos]){
if(x==par[pos])continue;
par[x]=pos;
dfs1(x);
spec[pos]|=spec[x];
}
if(spec[pos]){
specs.pb(pos);
}
}
void dfs2(int pos){
for(int x:komsu[pos]){
if(x==par[pos])continue;
if(spec[x])continue;
dp[pos]++;
dfs2(x);
res[pos].pb(dp[x]);
}
sort(res[pos].begin(),res[pos].end(),[&](const int &x,const int &y){return x>y;});
if(dp[pos]<=1){
return;
}
dp[pos]+=res[pos][1];
}
bool f(int x){
int used=0;
for(int i=0;i<m;i++){
int pos=specs[i];
int itr=0;
while(itr<res[pos].size()){
if(res[pos][itr]+ek[pos]+used>x)itr++;
else break;
}
if(itr==res[pos].size()){
if(ek[pos]+used>x)return false;
}
if(itr+used>i+1)return false;
used+=itr;
}
return true;
}
int main(){
ios_base::sync_with_stdio(false);cin.tie(NULL);
cin>>n>>t>>r;
spec[r]=1;
for(int i=1;i<n;i++){
int x,y;cin>>x>>y;
komsu[x].pb(y);
komsu[y].pb(x);
}
dfs1(t);
specs.pop_back();
m=specs.size();
for(int i:specs){
dfs2(i);
}
int l=0,r=n;
while(l<r){
int m=(l+r)/2;
if(f(m))r=m;
else l=m+1;
}
cout<<l<<endl;
}
