#include <bits/stdc++.h>
using namespace std;
using i64 = long long;
int n,A,B,par[300009],inpAB[300009],res=2e9;
vector<int> adj[300009];
struct pathVal
{
int v,s; vector<int> cs;
pathVal(int vv):cs(0){v=vv;s=0;}
};
vector<pathVal> pathAB;
void GetPar(int u,int pu)
{
par[u]=pu;
for(int& v:adj[u])
if(v!=pu) GetPar(v,u);
}
int calc(int u,int pu)
{
int mx=0; vector<int> tmp;
for(int& v:adj[u])
{
if(v==pu or inpAB[u]&inpAB[v]) continue;
tmp.push_back(calc(v,u));
}
sort(tmp.begin(),tmp.end(),greater<int>());
for(int i=0;i<tmp.size();++i) mx=max(mx,tmp[i]+i+1);
return mx;
}
int calc2(int p,bool AB) //0:A, 1:B
{
int mx=pathAB[p].s;
if(AB)
{
for(int i=p+1;i<pathAB.size();++i)
{
if(pathAB[i].cs.empty()) ++mx;
else
{
int tmp=0;
for(int ok=0,c=0,j=0;j<pathAB[i].cs.size();++j)
{
if(pathAB[i].cs[j]<mx and !ok) tmp=max(tmp,mx+(++c)),ok=1;
tmp=max(tmp,pathAB[i].cs[j]+(++c));
}
mx=tmp;
}
}
}
else
{
for(int i=p-1;i>=0;--i)
{
if(pathAB[i].cs.empty()) ++mx;
else
{
int tmp=0;
for(int ok=0,c=0,j=0;j<pathAB[i].cs.size();++j)
{
if(pathAB[i].cs[j]<mx and !ok) tmp=max(tmp,mx+(++c)),ok=1;
tmp=max(tmp,pathAB[i].cs[j]+(++c));
}
mx=tmp;
}
}
}
return mx;
}
int main()
{
if(fopen("TDL.INP","r"))
{
freopen("TDL.INP","r",stdin);
freopen("TDL.OUT","w",stdout);
}
else if(fopen("D:/CPP/THEMIS/test.inp","r"))
{
freopen("D:/CPP/THEMIS/test.inp","r",stdin);
freopen("D:/CPP/THEMIS/test.out","w",stdout);
}
ios_base::sync_with_stdio(0);
cin.tie(0);cout.tie(0);
cin>>n>>A>>B;
for(int u,v,i=1;i<n;++i)
{
cin>>u>>v;
adj[u].push_back(v);
adj[v].push_back(u);
}
GetPar(A,0);
for(int i=B;i!=A;i=par[i])
{
pathAB.push_back(pathVal(i));
inpAB[i]=1;
}
pathAB.push_back(pathVal(A)); inpAB[A]=1;
reverse(pathAB.begin(),pathAB.end());
for(int i=0;i<pathAB.size();++i)
{
pathAB[i].s=calc(pathAB[i].v,0);
for(int& u:adj[pathAB[i].v])
if(!inpAB[u]) pathAB[i].cs.push_back(calc(u,pathAB[i].v));
sort(pathAB[i].cs.begin(),pathAB[i].cs.end(),greater<int>());
}
for(int i=1;i<pathAB.size();++i)
res=min(res,max(calc2(i-1,0),calc2(i,1)));
cout<<res;
}
