#include<bits/stdc++.h>
#define N 300005
#define pb push_back
using namespace std;
const int inf=1e9;
int n,rt,A,B,up[N][20],dep[N];
vector<int>rd[N],d[N];
int f[N],g[N],h[N],dh[N],sz[N],dfn[N],low[N],son[N],len[N],fa[N];
int Mx[N],Mx1[N],mx[N],mx1[N];
void dfs(int u,int f){
	up[u][0]=fa[u]=f,son[u]=-1,d[dep[u]].pb(u),sz[u]=1;
	for(int i=1;(1<<i)<=dep[u];++i)up[u][i]=up[up[u][i-1]][i-1];
	Mx[u]=Mx1[u]=-1;
	for(int v:rd[u]){
		if(v==f)continue;
		dep[v]=dep[u]+1,dfs(v,u),sz[u]+=sz[v],len[u]=max(len[u],len[v]+1);
		if(son[u]==-1||sz[v]>sz[son[u]])son[u]=v;
		if(Mx[u]==-1)Mx[u]=v;else Mx1[u]=v;
	}
}
int LCA(int u,int v){
	if(dep[u]<dep[v])swap(u,v);
	for(int i=19;i>=0;--i)if(up[u][i]&&dep[up[u][i]]>=dep[v])u=up[u][i];
	if(u==v)return u;
	for(int i=19;i>=0;--i)if(up[u][i]!=up[v][i])u=up[u][i],v=up[v][i];
	return up[u][0];
}
int dis(int u,int v){return dep[u]+dep[v]-2*dep[LCA(u,v)];}
int getkth(int u,int k){
	for(int i=19;i>=0;--i)if((k>>i)&1)u=up[u][i];
	return u;
}
void upd(int p){
	int q=fa[p];
	if(Mx[q]==p)return;
	if(Mx1[q]==p){
		if(h[p]>h[Mx[q]])swap(Mx[q],Mx1[q]);
		return;
	}
	if(h[p]>=h[Mx[q]])Mx1[q]=Mx[q],Mx[q]=p;
	else if(Mx1[q]==-1||h[p]>=h[Mx1[q]])Mx1[q]=p;
}
int main(){
	ios::sync_with_stdio(0);
	cin>>n>>rt>>A>>B;
	for(int i=1;i<=n;++i)h[i]=0,dh[i]=inf;
	if(A<=B)cout<<1<<endl,exit(0);
	for(int i=1,a,b;i<n;++i)cin>>a>>b,rd[a].pb(b),rd[b].pb(a);
	dfs(rt,0);
	for(int i=1;i<=n;++i)if(len[i]<A)f[i]=inf;
	for(int i=1;i<=n;++i){
		if(len[i]<A)f[i]=min(f[i],i);
		for(int x=fa[i],lst=i,c=1;c<=B&&x;++c,x=fa[x],lst=fa[lst]){
			for(int v:rd[x])if(v!=fa[x]&&v!=lst)dh[v]=min(dh[v],i);
			if(len[x]<A)f[x]=min(f[x],i);
		}
	}
	for(int dd=n;dd>=0;--dd){
		if(dd+A<=n){
			for(int v:d[dd+A]){
				g[v]=inf;
				for(int i=1;i<=n;++i){
					if(dep[i]<dep[v]&&getkth(v,dep[v]-dep[i])==i)continue;
					if(dis(i,v)<=B)g[v]=min(g[v],f[i]);
				}
				for(int x=fa[v],lst=v,c=1;c<=B;++c,x=fa[x],lst=fa[lst]){
					int ans=0;
					if(Mx[x]!=lst)ans=h[Mx[x]];
					else if(Mx1[x]!=-1)ans=h[Mx1[x]];
					if(ans==0)ans=min(x,dh[lst]);
					g[v]=min(g[v],ans);
				}
				int q=getkth(v,A),p=getkth(v,A-1);
				f[q]=max(f[q],g[v]),h[p]=max(h[p],g[v]);
				upd(p);
			}
		}
	}cout<<f[rt];
	return 0;
}