//source: https://oj.uz/problem/view/COI16_torrent
#include<iostream>
#include<algorithm>
#include<vector>
using namespace std;
const int N=3e5+5;
int n,a,b;
vector<int> adj[N],ror;
void DFS(int pre,int u){
if(u==b) return;
for(auto i:adj[u]) if(i!=pre){
DFS(u,i);
if(i==ror.back()) ror.push_back(u);
}
}
int Cal(int pre,int u,int ban){
vector<int> v;
for(auto i:adj[u]){
if(i==pre||i==ban) continue;
v.push_back(Cal(u,i,ban));
}
if(v.empty()) return 0;
sort(begin(v),end(v),greater<int>());
int ma=0;
for(int i=0;i<(int)v.size();i++) ma=max(ma,v[i]+i);
return ma+1;
}
int main(){
cin.tie(0)->sync_with_stdio(0);
cin>>n>>a>>b;
for(int i=1,u,v;i<n;i++){
cin>>u>>v;
adj[u].push_back(v);
adj[v].push_back(u);
}
ror.push_back(b);
DFS(0,a);
int l=0, r=ror.size(), ans=N;
while(l<=r){
int mid=(l+r)>>1;
int v1=Cal(0,a,ror[mid]), v2=Cal(0,b,ror[mid+1]);
ans=min(ans,max(v1,v2));
if(v1>v2) l=++mid;
else r=--mid;
}
cout<<ans;
return 0;
}
