#include<bits/stdc++.h>
using namespace std;
template <typename T> inline void read(T &x)
{
x=0;T f=1;char c=getchar();
for(;!isdigit(c);c=getchar()) if(c=='-')f=-1;
for(;isdigit(c);c=getchar()) x=(x<<1)+(x<<3)+(c^48);
x*=f;
}
template <typename T,typename ...Args>void read(T &x,Args&...args){read(x),read(args...);}
template <typename T> void print(T x)
{
if(x<0) x=-x,putchar('-');
if(x>9) print(x/10);
putchar(x%10+48);
}
template <typename T> void print(T x,char c){print(x); putchar(c);}
template<typename T>inline void output(T x){print(x,' ');}
template<typename T,typename ...Arg>inline void output(T x,Arg ...arg){output(x);output(arg...);}
const int N=300007,inf=0x3f3f3f3f;
int n,rt[2],cnt,h[N],dp[N],pa[N],poss; vector<int>tmp,pos;
struct edge{int to,nxt;}mp[N<<1];
void add(int x,int y)
{
cnt++;
mp[cnt].nxt=h[x];
mp[cnt].to=y;
h[x]=cnt;
}
void prework(int x,int fa)
{
pa[x]=fa;
for(int i=h[x];i;i=mp[i].nxt)
{
int y=mp[i].to;
if(y==fa) continue;
prework(y,x);
}
tmp.clear();
for(int i=h[x];i;i=mp[i].nxt)
if(mp[i].to!=fa)
tmp.push_back(dp[mp[i].to]);
sort(tmp.begin(),tmp.end());
reverse(tmp.begin(),tmp.end());
for(int i=1;i<=tmp.size();i++)
dp[x]=max(dp[x],tmp[i-1]+i);
}
void init()
{
int now=rt[1];
while(now!=rt[0])
pos.push_back(now),now=pa[now];
}
int sol(int x)
{
if(x==poss) return -inf;
vector<int>vvv; int xxx=0;
for(int i=h[x];i;i=mp[i].nxt)
if(mp[i].to!=pa[x])
vvv.push_back(sol(mp[i].to));
sort(vvv.begin(),vvv.end());
reverse(vvv.begin(),vvv.end());
for(int i=1;i<=vvv.size();i++)
xxx=max(xxx,vvv[i-1]+i);
return xxx;
}
bool check(int x)
{
poss=-1; int tim=x;
for(int i=0;i<pos.size();i++)
{
tmp.clear(); int ok=1;
for(int j=h[pos[i]];j;j=mp[j].nxt)
if(mp[j].to!=pa[pos[i]]&&(i==0||mp[j].to!=pos[i-1]))
tmp.push_back(dp[mp[j].to]);
sort(tmp.begin(),tmp.end());
reverse(tmp.begin(),tmp.end());
for(int j=0;j<tmp.size();j++)
{
tmp[j]+=j+1;
if(tmp[j]>tim) ok=0;
}
if(!ok) break;
ok=tmp.size();
for(int j=tmp.size()-1;j>=0;j--)
{
if(tmp[j]==tim) break;
ok=j;
}
tim--; tim-=ok; poss=pos[i];
}
if(poss==-1) return false;
return sol(rt[0])<=x;
}
int main()
{
read(n,rt[0],rt[1]);
for(int i=1,x,y;i<n;i++)
read(x,y),add(x,y),add(y,x);
prework(rt[0],0); init();
int ml=0,mr=n,ans;
while(ml<=mr)
{
int mid=(ml+mr)>>1;
if(check(mid)) ans=mid,mr=mid-1;
else ml=mid+1;
}
print(ans,'\n');
return 0;
}
