#include <bits/stdc++.h>
#define debug cerr << "\033[32m[" << __LINE__ << "]\033[0m "
#define SZ(x) ((int) x.size() - 1)
#define all(x) x.begin(), x.end()
#define ms(x, y) memset(x, y, sizeof x)
#define F(i, x, y) for (int i = (x); i <= (y); i++)
#define DF(i, x, y) for (int i = (x); i >= (y); i--)
using namespace std;
typedef long long ll;
typedef unsigned long long ull;
template <typename T> T& chkmax(T& x, T y) {return x = max(x, y);}
template <typename T> T& chkmin(T& x, T y) {return x = min(x, y);}
template <typename T> T& read(T &x) {
	x = 0; int f = 1; char c = getchar();
	for (; !isdigit(c); c = getchar()) if (c == '-') f = - f;
	for (; isdigit(c); c = getchar()) x = (x << 1) + (x << 3) + (c ^ 48);
	return x *= f;
}
const int N = 3e5 + 10, inf = 1e9;
int n, R, a, b, dfn[N], nfd[N], rdfn[N], dfscnt, dep[N], mn[N], mmn[N], l[N], r[N];
vector <int> v[N], t[N];
priority_queue <int, vector <int>, greater <int>> q[N];
void predfs(int x, int fa) {
	mn[x] = x;
	nfd[dfn[x] = ++dfscnt] = x;
	t[dep[x] = dep[fa] + 1].push_back(dfn[x]);
	q[x].push(x);
	int mn1 = x, id = x, mn2 = inf;
	for (int i: v[x])
		if (i != fa) {
			predfs(i, x);
			while (dep[q[i].top()] - dep[x] > b) q[i].pop();
			chkmin(mn[x], q[i].top());
			if (q[i].top() < mn1) {
				mn2 = mn1;
				mn1 = q[i].top();
				id = i;
			} else chkmin(mn2, q[i].top());
			if (q[i].size() > q[x].size()) swap(q[i], q[x]);
			while (q[i].size()) {
				q[x].push(q[i].top());
				q[i].pop();
			}
		}
	for (int i: v[x])
		if (i != fa) mmn[i] = id == i ? mn2 : mn1;
	rdfn[x] = dfscnt;
	if (dep[x] + a - 1 <= n) l[x] = lower_bound(all(t[dep[x] + a - 1]), dfn[x]) - t[dep[x] + a - 1].begin(), r[x] = upper_bound(all(t[dep[x] + a - 1]), rdfn[x]) - t[dep[x] + a - 1].begin();
}
int k, p[N];
int tag[N << 2];
template<typename info_t,typename tag_t>struct Sgt{
#define ls(k) ((k)<<1)
#define rs(k) ((k)<<1|1)
	struct node{info_t info;tag_t tag;}t[N<<2];
	void apply(int k,tag_t v){t[k].info=t[k].info+v;t[k].tag=t[k].tag+v;}
	void push(int k){if(t[k].tag!=tag_t())apply(ls(k),t[k].tag),apply(rs(k),t[k].tag),t[k].tag=tag_t();}
	void pull(int k){t[k].info=t[ls(k)].info+t[rs(k)].info;}
	void clear(){
		for(int i=1;i<=4*n;i++)
			t[i].info=info_t(),t[i].tag=tag_t();
	}
	void modify(int l,int r,int k,int ql,int qr,tag_t v){
		if(ql>qr)return;
		if(ql<=l&&r<=qr)return apply(k,v);
		int mid=(l+r)>>1;push(k);
		if(ql<=mid)modify(l,mid,ls(k),ql,qr,v);
		if(mid<qr)modify(mid+1,r,rs(k),ql,qr,v);
		pull(k);
	}
	info_t query(int l,int r,int k,int ql,int qr){
		if(ql>qr)return info_t();
		if(ql<=l&&r<=qr)return t[k].info;
		int mid=(l+r)>>1;push(k);
		info_t res=info_t();
		if(ql<=mid){
			if(mid<qr)res=query(l,mid,ls(k),ql,qr)+query(mid+1,r,rs(k),ql,qr);
			else res=query(l,mid,ls(k),ql,qr);
		}else res=query(mid+1,r,rs(k),ql,qr);
		return res;
	}
#undef ls
#undef rs
};
struct info_t{int val;info_t(int _val=0):val(_val){}};
struct tag_t{int tag;tag_t(int _tag=0):tag(_tag){}};
info_t operator +(info_t lhs,info_t rhs){return info_t(max(lhs.val,rhs.val));}
info_t operator +(info_t lhs,tag_t rhs){return info_t(max(lhs.val,rhs.tag));}
tag_t operator +(tag_t lhs,tag_t rhs){return tag_t(max(lhs.tag,rhs.tag));}
bool operator !=(tag_t lhs,tag_t rhs){return lhs.tag!=rhs.tag;}
Sgt<info_t,tag_t>sgt;
void cdfs(int u,int pre,int mid){
	p[u]=inf;
	array<int,3>arr={inf,inf,-1};
	for(auto _v:v[u]){
		if(_v==pre)continue;
		cdfs(_v,u,mid);
		p[u]=min(p[u],p[_v]+1);
		if(p[_v]<arr[0])swap(arr[0],arr[1]),arr[0]=p[_v],arr[2]=_v;
		else arr[1]=min(arr[1],p[_v]);
	}
	array<int,2>son={0,0},key={0,0};
	for(auto _v:v[u]){
		if(_v==pre)continue;
		if(l[_v]<r[_v]){
			son[1]=son[0],son[0]=_v;
			for(int j=l[_v];j<r[_v];j++){
				int pos=nfd[t[dep[_v]+a-1][j]];
				if(p[pos]>b&&dep[pos]>sgt.query(1,n,1,dfn[pos],dfn[pos]).val){
					key[1]=key[0],key[0]=_v;
					break;
				}
			}
		}
		sgt.modify(1,n,1,dfn[_v],rdfn[_v],dep[u]+b-arr[arr[2]==_v]-1);
	}
	for(auto _v:v[u]){
		if(_v==pre)continue;
		if(!son[son[0]==_v]){
			if(mmn[_v]<=mid)
				sgt.modify(1,n,1,dfn[_v],rdfn[_v],dep[u]+b);
		}else if(!key[key[0]==_v]){
			sgt.modify(1,n,1,dfn[_v],rdfn[_v],dep[u]+b);
		}
	}
	if(!key[0]){
		if(mn[u]<=mid)
			p[u]=0;
	}else if(!son[0]){
		p[u]=0;
	}
}
bool check() {
	sgt.clear();
	cdfs(R, 0, k);
	return p[R] == 0;
}
signed main() {
	read(n), read(R), read(a), read(b);
	if (a <= b) {
		cout << 1;
		return 0;
	}
	F(i, 1, n - 1) {
		int x, y; read(x), read(y);
		v[x].push_back(y);
		v[y].push_back(x);
	}
	predfs(R, 0);
	// for(int i=1;i<=n;i++){
	// 	cout<<dfn[i]<<' '<<rdfn[i]<<' '<<mn[i]<<' '<<mmn[i]<<' '<<l[i]<<' '<<r[i]<<'\n';
	// }
	int l = 0, r = n;
	while (l + 1 < r) {
		k = (l + r) >> 1;
		if (check()) r = k;
		else l = k;
	}
	cout << r;
	return 0;
}
/* why?
*/