#include<bits/stdc++.h>
using namespace std;
const int MAXN=1e6+67;
const int INF=1e9;
vector<int> ds[MAXN],vi[MAXN];
int dp[MAXN],root[MAXN],pref[MAXN];
bool del[MAXN];
void dfs(int i,int pre)
{
root[i]=pre;
dp[i]=ds[i].size()-(i!=pre);
int mx1=0,mx2=0;
for(auto v:ds[i]) if(v!=pre)
{
dfs(v,i);
if(mx1<dp[v]) mx2=mx1,mx1=dp[v];
else if(mx2<dp[v]) mx2=dp[v];
}
dp[i]+=mx2;
}
int main()
{
ios::sync_with_stdio(0);
cin.tie(0);
cout.tie(0);
int n,t,m;
cin>>n>>t>>m;
for(int i=1;i<n;i++)
{
int l,r;
cin>>l>>r;
ds[l].push_back(r),ds[r].push_back(l);
}
dfs(t,t);
int pos=m,sum=0;
vector<int> vv;
while(pos!=t)
{
for(auto v:ds[pos]) if(v!=root[pos]&&!del[v]) vi[pos].push_back(dp[v]);
vv.push_back(pos),del[pos]=true,pos=root[pos];
}
while(!vv.empty()) pref[vv.back()]=(sum+=vi[vv.back()].size()),vv.pop_back();
int l=0,r=n*2,ans=n*2;
while(l<=r)
{
int mid=(l+r)/2,res=m,cnt=1,z=0;
bool ck=true;
while(res!=t)
{
int k=0;
for(int j=0;j<vi[res].size();j++) if(vi[res][j]+pref[res]+z>mid)
{
cnt--,k++;
if(cnt<0)
{
ck=false;
break;
}
}
cnt++,z+=k,res=root[res];
}
if(ck&&z<=mid) r=mid-1,ans=mid;
else l=mid+1;
}
cout<<ans;
}
