#include<iostream>
#include<cstdio>
#include<vector>
#define N 300000
using namespace std;
const int inf=(int)(1e9);
int read()
{
	char c=0;
	int sum=0;
	while (c<'0'||c>'9') c=getchar();
	while ('0'<=c&&c<='9') sum=sum*10+c-'0',c=getchar();
	return sum;
}
int n,sr,A,B,d,lengs,leng[N+1],sleng[N+1],lg[N+1],depth[N+1],minn[N+1],dfn[N+1],sz[N+1],sfa[N+1],tong[N+1],length,dp[N+1];
bool vis[N+1],vst[N+1];
vector<int>E[N+1];
vector<int>ES[N+1];
vector<int>p[N+1];
void add(int x,int y)
{
	E[x].push_back(y),E[y].push_back(x);
	return;
}
void dfs(int x)
{
	dfn[x]=++lengs,sz[x]=1,tong[++length]=x;
	if (length>A) sfa[x]=tong[length-A+1],p[tong[length-A]].push_back(x);
	for (int i=0;i<E[x].size();++i)
		if (!dfn[E[x][i]])
			ES[x].push_back(E[x][i]),depth[E[x][i]]=depth[x]+1,dfs(E[x][i]),sz[x]+=sz[E[x][i]];
	--length;
	return;
}
struct seg
{
	struct node
	{
		int l,r,lazy;
	};
	node tree[(N<<2)+1];
	void build(int k,int l,int r)
	{
		tree[k].l=l,tree[k].r=r,tree[k].lazy=inf;
		if (l==r) return;
		int mid=(l+r)>>1;
		build(k<<1,l,mid),build(k<<1|1,mid+1,r);
		return;
	}
	void add(int k,int l,int r,int d)
	{
		if (tree[k].l==l&&tree[k].r==r)
		{
			tree[k].lazy=min(tree[k].lazy,d);
			return;
		}
		int mid=(tree[k].l+tree[k].r)>>1;
		if (r<=mid) add(k<<1,l,r,d);
		else if (l>=mid+1) add(k<<1|1,l,r,d);
		else add(k<<1,l,mid,d),add(k<<1|1,mid+1,r,d);
		return;
	}
	int query(int k,int x)
	{
		if (tree[k].l==tree[k].r) return tree[k].lazy;
		int mid=(tree[k].l+tree[k].r)>>1;
		if (x<=mid) return min(tree[k].lazy,query(k<<1,x));
		else return min(tree[k].lazy,query(k<<1|1,x));
	}
};
seg T;
bool query(int x,int y)
{
	return !(leng[y]<=B||T.query(1,dfn[y])+depth[y]<=B);
}
void dfs2(int x)
{
	int cnt=0,cnt2=0,sx,sminn=inf,rt=0;
	leng[x]=sleng[x]=inf;
	for (int i=0;i<ES[x].size();++i)
	{
		dfs2(ES[x][i]),sleng[x]=min(sleng[x],sleng[ES[x][i]]+1);
		if (leng[ES[x][i]]+1<leng[x]) sminn=leng[x],leng[x]=leng[ES[x][i]]+1,rt=ES[x][i];
		else sminn=min(sminn,leng[ES[x][i]]+1);
	}
	if (x<d) sleng[x]=0;
	if (p[x].empty())
	{
		dp[x]=sleng[x]>B;
		if (sleng[x]<=B)
		{
			for (int i=0;i<ES[x].size();++i) T.add(1,dfn[ES[x][i]],dfn[ES[x][i]]+sz[ES[x][i]]-1,-depth[x]);
		}
	}
	else
	{
		dp[x]=sx=0;
		for (int i=0;i<p[x].size();++i)
		{
			if (query(x,p[x][i])) dp[x]=vis[sfa[p[x][i]]]=1;
			vst[sfa[p[x][i]]]=1;
		}
		for (int i=0;i<ES[x].size();++i) cnt+=vst[ES[x][i]],cnt2+=vis[ES[x][i]];
		if (cnt==1)
		{
			if (!cnt2)
			{
				for (int i=0;i<ES[x].size();++i)
					if (!vst[ES[x][i]])
						T.add(1,dfn[ES[x][i]],dfn[ES[x][i]]+sz[ES[x][i]]-1,-depth[x]);
			}
			int res=inf;
			for (int i=0;i<ES[x].size();++i)
				if (!vst[ES[x][i]])
					res=min(res,sleng[ES[x][i]]+1);
			if (x<d||res<=B)
			{
				for (int i=0;i<ES[x].size();++i)
					if (vst[ES[x][i]])
						T.add(1,dfn[ES[x][i]],dfn[ES[x][i]]+sz[ES[x][i]]-1,-depth[x]);
			}
		}
		else
		{
			if (!cnt2)
			{
				for (int i=0;i<ES[x].size();++i) T.add(1,dfn[ES[x][i]],dfn[ES[x][i]]+sz[ES[x][i]]-1,-depth[x]);
			}
			if (cnt2==1)
			{
				for (int i=0;i<ES[x].size();++i)
					if (vis[ES[x][i]])
						T.add(1,dfn[ES[x][i]],dfn[ES[x][i]]+sz[ES[x][i]]-1,-depth[x]);
			}
		}
	}
	for (int i=0;i<ES[x].size();++i)
	{
		if (ES[x][i]==rt) T.add(1,dfn[ES[x][i]],dfn[ES[x][i]]+sz[ES[x][i]]-1,sminn-depth[x]);
		else T.add(1,dfn[ES[x][i]],dfn[ES[x][i]]+sz[ES[x][i]]-1,leng[x]-depth[x]);
	}
	if (!dp[x]) leng[x]=0;
	return;
}
bool check(int x)
{
	for (int i=1;i<=n;++i) minn[i]=inf,vis[i]=vst[i]=0;
	d=x,T.build(1,1,n),dfs2(sr);
	return dp[sr];
}
int main()
{
	int x,y,res=0;
	for (int i=2;i<=N;++i) lg[i]=lg[i>>1]+1;
	n=read(),sr=read(),A=read(),B=read();
	if (A<=B)
	{
		puts("1");
		return 0;
	}
	for (int i=1;i<=n-1;++i) x=read(),y=read(),add(x,y);
	depth[sr]=1,dfs(sr);
	for (int i=lg[n];i>=0;--i)
		if (res+(1<<i)<=n&&check(res+(1<<i)))
			res+=(1<<i);
	printf("%d\n",res);
	return 0;
}