#include <bits/stdc++.h>
#define rep(i,j,k) for(int i=(j);i<=(k);i++)
#define per(i,j,k) for(int i=(j);i>=(k);i--)
#define uint unsigned int
#define ll long long
#define ull unsigned long long
#define db double
#define ldb long double
#define pii pair<int,int>
#define pll pair<ll,ll>
#define mkp make_pair
#define eb emplace_back
#define SZ(S) (int)S.size()
//#define mod 998244353
//#define mod 1000000007
#define inf 0x3f3f3f3f
#define linf 0x3f3f3f3f3f3f3f3f
using namespace std;

template <class T>
void chkmax(T &x,T y){
	if(x < y) x = y;
}

template <class T>
void chkmin(T &x,T y){
	if(x > y) x = y;
}

inline int popcnt(int x){
	return __builtin_popcount(x);
}

inline int ctz(int x){
	return __builtin_ctz(x);
}


/*ll power(ll p,int k = mod - 2){
	ll ans = 1;
	while(k){
		if(k % 2 == 1) ans = ans * p % mod;
		p = p * p % mod;
		k /= 2;	
	}
	return ans;
}*/
int n,rt,A,B;
vector <int> G[3005];

int dp[2][3005],fa[3005],dfn[3005],siz[3005],dep[3005],N;//[0] 切掉向父亲,[1] 在父亲,切掉两条
int dis[305][305],val[3005];
int anc(int u,int v){
	return dfn[u] <= dfn[v] && dfn[v] < dfn[u] + siz[u];
}

void dfs0(int u,int f,int s){
	for(int v:G[u]){
		if(v == f) continue;
		dis[s][v] = dis[s][u] + 1;
		dfs0(v,u,s);
	}
}

void dfs(int u,int f){
	fa[u] = f;
	dep[u] = dep[f] + 1;
	dfn[u] = ++N;
	siz[u] = 1;
	for(int v:G[u]){
		if(v == f) continue;
		dfs(v,u);
		siz[u] += siz[v];
	}
}

void eval(int v){
	val[v] = dp[0][v];
	int p = v;
	rep(k,0,B - 1){
		chkmin(val[v],dp[1][p]);
		p = fa[p];
	}
	rep(w,1,n) if(dis[v][w] <= B && !anc(w,v)) chkmin(val[v],dp[0][w]);	
}

void dfs2(int u){
	for(int v:G[u]) if(v != fa[u]) dfs2(v);

	int valid = 0;
	rep(v,1,n){
		if(anc(u,v) && dep[v] - dep[u] == A){
			valid = 1;
			eval(v);
			chkmax(dp[0][u],val[v]);
		}
	}
	if(!valid){
		dp[0][u] = n + 1;
		rep(v,1,n) if(anc(u,v) && dep[v] - dep[u] <= B) chkmin(dp[0][u],v);
	}

	for(int vv:G[u]){
		if(vv == fa[u]) continue;

		valid = 0;
		rep(v,1,n){
			if(anc(u,v) && !anc(vv,v) && dep[v] - dep[u] == A){
				valid = 1;
				chkmax(dp[1][vv],val[v]);
			}
		}		
		if(!valid){
			dp[1][vv] = n + 1;
			rep(v,1,n) if(anc(u,v) && !anc(vv,v) && dep[v] - dep[u] <= B) chkmin(dp[1][vv],v);
		}
	}		
}

int main(){	
//	freopen("test.in","r",stdin);
	scanf("%d%d%d%d",&n,&rt,&A,&B);
	if(A <= B){
		printf("1\n");
		return 0;
	}
	rep(i,1,n - 1){
		int u,v;
		scanf("%d%d",&u,&v);
		G[u].eb(v);G[v].eb(u);
	}
	rep(u,1,n) dfs0(u,0,u);
	dfs(rt,0);
	dfs2(rt);
	printf("%d\n",dp[0][rt]);
	return 0;
}