#include<iostream>
#include<algorithm>
#include<iomanip>
#include<cmath>
#include<cstring>
#include<vector>
#include<queue>
#include<stack>
#include<tuple>
#include<set>
#include<map>
#include<random>
#include<chrono>
#include<array>
using namespace std;
const int MAX_N=3e5+5;
vector<int>g[MAX_N];
int a,b;
int n;
int parent[MAX_N];
bool used[MAX_N];
void initdfs(int u,int par)
{
for(int v:g[u])
{
if(v==par)continue;
parent[v]=u;
initdfs(v,u);
}
}
void paint(int u)
{
used[u]=1;
for(int v:g[u])
{
if(v==parent[u])continue;
paint(v);
}
}
int dfs(int u,int par,bool wh)
{
vector<int>mins;
for(int v:g[u])
{
if(v==par or used[v]!=wh)continue;
mins.push_back(dfs(v,u,wh));
}
if(mins.size()==0)return 0;
sort(mins.rbegin(),mins.rend());
int ans=0;
for(int i=0;i<mins.size();i++)
{
ans=max(ans,mins[i]+i+1);
}
return ans;
}
signed main ()
{
ios_base::sync_with_stdio(0);
cin.tie(NULL);
cout.tie(NULL);
cin>>n>>a>>b;
for(int i=1;i<n;i++)
{
int u,v;
cin>>u>>v;
g[u].push_back(v);
g[v].push_back(u);
}
initdfs(a,0);
int u=b;
int ans=1e9;
int z=-1;
while(u!=a)
{
paint(u);
if(u==b)ans=min(ans,max(dfs(a,0,0),dfs(b,0,1)));
if(dfs(a,0,0)>dfs(b,0,1))
{
if(z!=0 && z!=-1){ans=min(ans,dfs(a,0,0));break;}
z=0;
}
else
{
if(z!=1 && z!=-1){ans=min(ans,dfs(b,0,1));break;}
z=1;
}
u=parent[u];
}
cout<<ans<<"\n";
return 0;
}
