//code by Emissary
#include<bits/stdc++.h>

#define fi first
#define se second
#define vc vector
#define db double
#define ll long long
#define mk make_pair
#define pb push_back
#define PI pair<int,int>
#define ull unsigned long long
#define err cerr << "   -_-   " << endl
#define debug cerr << " ------------------- " << endl

#define input(x) freopen(#x".in","r",stdin)
#define output(x) freopen(#x".out","w",stdout)

#define NO puts("No")
#define YES puts("Yes")

//#define int long long

using namespace std;

namespace IO{
	inline int read(){
		int X=0, W=0; char ch=getchar();
		while(!isdigit(ch)) W|=ch=='-', ch=getchar();
		while(isdigit(ch)) X=(X<<1)+(X<<3)+(ch^48), ch=getchar();
		return W?-X:X;
	}
	inline void write(int x){
		if(x<0) x=-x, putchar('-');
		if(x>9) write(x/10);
		putchar(x%10+'0');
	}
	inline void sprint(int x){write(x), putchar(32);}
	inline void eprint(int x){write(x), putchar(10);}
}using namespace IO;

const int MAXN = 6e5+5;
const int inf = 1e9;

int siz[MAXN], tot;
int stk[MAXN], topf, up[MAXN], ups[MAXN];
int de[MAXN], toid[MAXN], dfn[MAXN], maxd[MAXN];
int n, r, a, b, val[MAXN], fa[MAXN], Max[MAXN<<2];
int head[MAXN], ne[MAXN<<1], to[MAXN<<1], minn[MAXN<<2], cnt;

vc<int> d[MAXN], son[MAXN];

int dp1[MAXN], dp2[MAXN], dp3[MAXN];

inline void add(int x, int y){++cnt;to[cnt]=y;ne[cnt]=head[x];head[x]=cnt;}

inline void dfs(int x, int f){
	stk[++topf]=x; siz[x]=1;
	if(topf>=a+1) up[x]=stk[topf-a];
	if(topf>=a) ups[x]=stk[topf-a+1];
	de[x]=de[fa[x]=f]+1; dfn[x]=++tot; toid[tot]=x;
	for(int i=head[x];i;i=ne[i]){
		if(to[i]==f) continue;
		dfs(to[i],x); son[x].pb(to[i]);
		maxd[x]=max(maxd[x],maxd[to[i]]+1);
		siz[x]+=siz[to[i]];
	}
	d[de[x]].pb(x); --topf;
	return ;
}

inline void build(int p, int l, int r){
	Max[p]=-inf;
	if(l==r) return minn[p]=val[toid[l]]?de[toid[l]]:inf, void();
	int mid=(l+r)>>1;
	build(p<<1,l,mid); build(p<<1|1,mid+1,r);
	return minn[p]=min(minn[p<<1],minn[p<<1|1]), void();
}

inline int chk(int p, int l, int r, int pos){
	//cerr << "de:" << de[toid[pos]] << ' ' << Max[p] << endl;
	if(de[toid[pos]]<=Max[p]) return 1;
	if(l==r) return 0;
	int mid=(l+r)>>1;
	if(pos<=mid) return chk(p<<1,l,mid,pos);
	return chk(p<<1|1,mid+1,r,pos);
}

inline void chkmax(int p, int l, int r, int L, int R, int v){
	if(L>R) return ;
	if(L<=l && r<=R) return Max[p]=max(Max[p],v), void();
	int mid=(l+r)>>1;
	if(L<=mid) chkmax(p<<1,l,mid,L,R,v);
	if(R>mid) chkmax(p<<1|1,mid+1,r,L,R,v);
	return ;
}

inline int ask(int p, int l, int r, int L, int R){
	if(L<=l && r<=R) return minn[p];
	int mid=(l+r)>>1, res=inf;
	if(L<=mid) res=min(res,ask(p<<1,l,mid,L,R));
	if(R>mid) res=min(res,ask(p<<1|1,mid+1,r,L,R));
	return res;
}

inline void upd(int p, int l, int r, int pos, int v){
	if(l==r) return minn[p]=v, void();
	int mid=(l+r)>>1;
	if(pos<=mid) upd(p<<1,l,mid,pos,v);
	else upd(p<<1|1,mid+1,r,pos,v);
	minn[p]=min(minn[p<<1],minn[p<<1|1]);
	return ;
}

inline bool check(int x){
//	cerr << "check:" << x << ' ' << maxd[r] << endl;
	for(int i=1;i<=n;++i) val[i]=i<=x; build(1,1,n);
	for(int i=1;i<=n;++i) dp1[i]=dp2[i]=1, dp3[i]=0;
	for(int i=n;i>=1;--i){
		for(auto j:d[i]){
			if(val[j]==1) chkmax(1,1,n,dfn[j],dfn[j]+siz[j]-1,de[j]+b);
			for(auto k:son[j]){
				int l=dfn[j], r=dfn[j]+siz[j]-1;
				int o=ask(1,1,n,dfn[k],dfn[k]+siz[k]-1);
				if(val[j]==1) o=de[j]; int dis=o-de[j]; dis=b-dis+de[j];
				chkmax(1,1,n,l,dfn[k]-1,dis), chkmax(1,1,n,dfn[k]+siz[k],r,dis);
				//cerr << "lca=" << j << ' ' << k << ' ' << dis << endl;
			}
		}
	}
	//err;
	for(int i=1;i<=n;++i){
		if(maxd[i]<a){
			int u=chk(1,1,n,dfn[i]);
			dp1[i]=dp3[i]=chk(1,1,n,dfn[i]);
			int p=ask(1,1,n,dfn[i],dfn[i]+siz[i]-1);	
			if(p-de[i]<=b) u=1;
			for(auto j:son[i]) dp2[j]=u; dp1[i]=dp3[i]=u;
			//if(dp3[i]==1) cerr << "node:" << i << ' ' << u << endl;
		}
		if(son[i].size()==1){
			int ver=son[i][0];
			int u=chk(1,1,n,dfn[i]);
			int p=ask(1,1,n,dfn[i],dfn[i]+siz[i]-1);
			if(p-de[i]<=b) u=1;	 dp2[ver]=u;
		}
	}
	//err;
	for(int i=1;i<=n<<2;++i) minn[i]=inf, Max[i]=-inf;
	//err;
	for(int i=n, e=i+b-1;e>=1;--i, --e){
		if(i>=1){
			for(auto j:d[i]){
				if(dp2[j]==1) chkmax(1,1,n,dfn[j],dfn[j]+siz[j]-1,de[j]+b-1);
				for(auto k:son[j]){
					int l=dfn[j], r=dfn[j]+siz[j]-1;
					int o=ask(1,1,n,dfn[k],dfn[k]+siz[k]-1);
					if(val[j]==1) o=de[j]; int dis=o-de[j]; dis=b-dis+de[j];
					chkmax(1,1,n,l,dfn[k]-1,dis), chkmax(1,1,n,dfn[k]+siz[k],r,dis);
				}
				if(dp1[j]==1) upd(1,1,n,dfn[j],de[j]);
			}
		}
		for(auto j:d[e]){
			dp3[j]|=chk(1,1,n,dfn[j]);
			int u=ask(1,1,n,dfn[j],dfn[j]+siz[j]-1);
			if(u-de[j]<=b) dp3[j]=1;
			if(dp3[j]==0){
				if(up[j]) dp1[up[j]]=0;
				if(ups[j]) dp2[ups[j]]=0;
			}
		}
	}
//	for(int i=1;i<=n;++i){
//		cout << "info:" << i << endl;
//		cout << dp1[i] << ' ' << dp2[i] << ' ' << dp3[i] << endl; 
//	} 
	return dp1[r]==1;
}

int main(){
//	input(aba); output(aba);
	freopen("25.in","r",stdin);
	n=read(), r=read(), a=read(), b=read();
	if(a<=b) return eprint(1), 0;
	for(int i=2;i<=n;++i){
		int x, y;
		x=read(), y=read();
		add(x,y), add(y,x);
	}
	err;
	dfs(r,0);
	err;
	int l=1, r=n, mid;
	while(l<=r){
		mid=(l+r)>>1;
//		cerr << "range:" << l << ' ' << r << ' ' << mid << endl;
		if(check(mid)) r=mid-1;
		else l=mid+1;
	}
	eprint(l);
	return 0;
}

/*
9 6 2 1
1 3
1 6
2 4
2 5
2 7
3 9
4 6
4 8
*/