// MagicDark
#include <bits/stdc++.h>
#define mid ((l + r) >> 1)
#define ls num << 1
#define rs ls | 1
#define li ls, l, mid
#define ri rs, mid + 1, r
#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];//, tl[N], tr[N];
vector <int> v[N], t[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]);
	int mn1 = x, id = x, mn2 = inf;
	for (int i: v[x])
		if (i != fa) {
			predfs(i, x), chkmin(mn[x], mn[i]);
			if (mn[i] < mn1) {
				mn2 = mn1;
				mn1 = mn[i];
				id = i;
			} else chkmin(mn2, mn[i]);
		}
	for (int i: v[x])
		if (i != fa) mmn[i] = id == i ? mn2 : mn1;
	rdfn[x] = dfscnt;
	// if (dep[x] + a <= n) l[x] = lower_bound(all(t[dep[x] + a]), dfn[x]) - t[dep[x] + a].begin(), r[x] = lower_bound(all(t[dep[x] + a]), rdfn[x]) - t[dep[x] + a].begin();
	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];
void change(int num, int l, int r, int L, int R, int x) {
	if (L <= l && r <= R) return chkmax(tag[num], x), void();
	if (mid >= L) change(li, L, R, x);
	if (mid < R) change(ri, L, R, x);
}
int query(int num, int l, int r, int x) {
	if (l == r) return tag[num];
	if (mid >= x) return max(query(li, x), tag[num]);
	return max(query(ri, x), tag[num]);
}
void dfs(int x, int fa) {
	p[x] = inf;
	int mn1 = inf, id, mn2 = inf;
	for (int i: v[x])
		if (i != fa) {
			dfs(i, x), chkmin(p[x], p[i] + 1);
			if (p[i] < mn1) {
				mn2 = mn1;
				mn1 = p[i];
				id = i;
			} else chkmin(mn2, p[i]);
		}
	bool flag = true;//, gg = true;
	int id1 = 0, id2 = 0;
	int iid1 = 0, iid2 = 0;
	for (int i: v[x])
		if (i != fa) {
			if (l[i] < r[i]) {
				iid2 = iid1;
				iid1 = i;
				// gg = false;
				F(j, l[i], r[i] - 1) {
					int e = nfd[t[dep[x] + a][j]];
					// assert(j < t[dep[x] + a].size());
					// debug << dep[x] + a << endl;
					// if (x == 6) 
					// debug << e << " " << dep[e] << " " << p[e] << " " << dep[e] << " " << query(1, 1, n, dfn[e]) << endl;
					if (p[e] > b && dep[e] > query(1, 1, n, dfn[e])) {
						// if (x == R) debug << i << " " << e << endl;
						id2 = id1, id1 = i;
						break;
					}
				}
			}
			change(1, 1, n, dfn[i], rdfn[i], dep[x] + b - (id == i ? mn2 : mn1) - 1);
		}
	for (int i: v[x])
		if (i != fa) {
			bool flag;
			if (!(iid1 == i ? iid2 : iid1)) flag = mmn[i] <= k;
			else flag = !(id1 == i ? id2 : id1);
			if (flag) change(1, 1, n, dfn[i], rdfn[i], dep[x] + b);
		}
	if (!iid1) flag = mn[x] <= k;
	else flag = !id1;
	if (flag) p[x] = 0;
	// debug << x << " " << flag << endl;// << " " << mn[x] << " " << iid1 << " " << k << endl;
}
bool check() {
	F(i, 1, n * 4) tag[i] = 0;
	dfs(R, 0);
	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);
	// k = 1;
	// cout << check() << endl;
	// return 0;
	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?
*/