#include<bits/stdc++.h>
using namespace std;
#define ll long long
namespace IO{//by cyffff
    int len=0;
    char ibuf[(1<<21)+1],*iS,*iT,out[(1<<25)+1];
    #if ONLINE_JUDGE
    #define gh() (iS==iT?iT=(iS=ibuf)+fread(ibuf,1,(1<<21)+1,stdin),(iS==iT?EOF:*iS++):*iS++)
    #else
    #define gh() getchar()
    #endif
    #define reg register
    inline int read(){
        reg char ch=gh();
        reg int x=0;
        reg char t=0;
        while(ch<'0'||ch>'9')   t|=ch=='-',ch=gh();
        while(ch>='0'&&ch<='9') x=x*10+(ch^48),ch=gh();
        return t?-x:x;
    }
    inline void putc(char ch){
        out[len++]=ch;
    }
    template<class T>
    inline void write(T x){
        if(x<0)putc('-'),x=-x;
        if(x>9)write(x/10);
        out[len++]=x%10+48;
    }
    inline void flush(){
        fwrite(out,1,len,stdout);
        len=0;
    }
    inline char getc(){
        char ch=gh();
        while(ch<'A'||ch>'Z') ch=gh();
        return ch;
    }
}
using IO::read;
using IO::write;
using IO::flush;
using IO::getc;
using IO::putc;
#define pii pair<int,int>
#define mpr make_pair
#define fir first
#define sec second
const int N=3e5+10,INF=1e9;
int n,r,A,B,fa[N],dep[N],dfn[N],low[N],idfn[N],tim,p[N],stk[N],top;
int f[N],g[N],h[N],mxd,d0[N],d1[N];
vector<int>a[N],b[N],sl[N],tl[N],sd[N];
inline void add(int u,int v){
    a[u].push_back(v);
}
inline void dfs(int x,int fa){
    idfn[dfn[x]=++tim]=x,::fa[x]=fa;
    stk[++top]=x,dep[x]=dep[fa]+1,mxd=max(mxd,dep[x]);
    if(top>A) sl[stk[top-A]].push_back(x);
    if(top>=A) tl[stk[top-A+1]].push_back(x);
    sd[dep[x]].push_back(x);
    for(auto t:a[x]){
        if(t==fa) continue;
        dfs(t,x),b[x].push_back(t);
    }
    low[x]=tim,top--;
//    printf("%d %d %d %d\n",x,dfn[x],low[x],dep[x]);
}
struct Segment_Tree{
	#define ls (rt<<1)
	#define rs (rt<<1|1)
	int tag[N<<2];
	inline void pushtag(int rt,int v){
		tag[rt]=min(tag[rt],v);
	}
	inline void build(int rt,int l,int r){
		tag[rt]=INF;
		if(l==r) return ;
		int mid=l+r>>1;
		build(ls,l,mid),build(rs,mid+1,r);
	}
	inline void modify(int rt,int l,int r,int L,int R,int v){
		if(L<=l&&r<=R) return pushtag(rt,v);
		int mid=l+r>>1;
		if(L<=mid) modify(ls,l,mid,L,R,v);
		if(R>mid) modify(rs,mid+1,r,L,R,v);
	}
	inline int query(int rt,int l,int r,int p){
		if(l==r) return tag[rt];
		int mid=l+r>>1,ans=tag[rt];
		if(p<=mid) ans=min(ans,query(ls,l,mid,p));
		else ans=min(ans,query(rs,mid+1,r,p));
		return ans;
	}
}T;
inline bool check(int x){//最终是否会走到 <=x 的点 A 最大化 B 最小化 
//	printf("---check(%d)---\n",x);
	T.build(1,1,n);
	for(int d=mxd;d;d--){
		if(d+A<=mxd)
			for(auto i:sd[d+A])
				g[i]=min(d1[i],dep[i]+T.query(1,1,n,dfn[i]))<=B;//,printf("g[%d]=%d\n",i,g[i]);
		for(auto i:sd[d]){
			d0[i]=i<=x?0:INF;
			int cnt=i<=x,cg=0;
			for(auto j:b[i])
				d0[i]=min(d0[i],d0[j]+1),cnt+=d0[j]<B;
			if(!sl[i].size()) f[i]=d0[i]<=B;
			else{
				f[i]=1;
				for(auto j:sl[i])
					f[i]&=g[j],cg+=!g[j];
			}
//			printf("f[%d]=%d\n",i,f[i]);
			for(auto j:b[i]){
				if(sl[i].size()==tl[j].size()) h[j]=cnt>=2||(cnt&&d0[j]>=B);//,printf("! %d %d %d\n",i,j,d0[j]>=B);
				else{
					int tg=0;
					for(auto k:tl[j])
						tg+=!g[k];
					h[j]=tg==cg;
				}
			}
			d1[i]=f[i]?0:INF;
			for(auto j:b[i])
				d1[i]=min(d1[i],d1[j]+1);
			int pr=INF,sf=INF;
			for(int j=0;j<b[i].size();j++)
				T.modify(1,1,n,dfn[b[i][j]],low[b[i][j]],min(pr,h[b[i][j]]?-d:INF)),pr=min(pr,d1[b[i][j]]+1);
			for(int j=b[i].size()-1;j>=0;j--)
				T.modify(1,1,n,dfn[b[i][j]],low[b[i][j]],min(sf,h[b[i][j]]?-d:INF)),sf=min(sf,d1[b[i][j]]+1);
		}
	}
	return f[r];
}
int main(){
    n=read(),r=read(),A=read(),B=read();
    if(A<=B) return puts("1"),0;
    for(int i=1;i<n;i++){
        int u=read(),v=read();
        add(u,v),add(v,u);
    } 
    dfs(r,r);
    int L=1,R=n,ans=n+1;
    while(L<=R){
        int mid=L+R>>1;
        if(check(mid)) R=mid-1,ans=mid;
        else L=mid+1;
    }
    write(ans);
    flush();
}