思路是对深度维护 set，启发式合并。

#include <bits/stdc++.h>
#define int long long
#define set multiset

using namespace std;

const int maxn = 1e6 + 10;

int n, k, rt;
vector<int> g[maxn];
int deg[maxn];
int dep[maxn];
set<int> p[maxn];

void dfs(int u, int fa) {
	dep[u] = dep[fa] + 1;
	for (int v : g[u]) {
		if (v == fa) continue;
		dfs(v, u);
	}
}

set<int> merge(set<int> u, set<int> v) {
	if (u.size() < v.size()) swap(u, v);
	while (v.size()) {
		int x = *v.begin();
		v.erase(x), u.insert(x);
	}
	return u;
}

void solve(int u, int fa) {
	if (deg[u] == 1) return;
	for (int v : g[u]) {
		if (v == fa) continue;
		solve(v, u);
	}
	set<int> res = set<int> (), nw = set<int> ();
	for (int v : g[u]) res = merge(res, p[v]);
	int x, y = *res.begin();
	while (res.size() >= 2) {
		x = *res.begin(), res.erase(res.find(x));
		y = *res.begin();
		if (x + y - dep[u] * 2 > k) nw.insert(x);
	}
	nw.insert(y);
	p[u] = nw;
}

signed main() {
	cin >> n >> k;
	for (int i = 1, u, v; i < n; i++) {
		cin >> u >> v;
		g[u].push_back(v), g[v].push_back(u);
		deg[u]++, deg[v]++;
	}
	for (int i = 1; i <= n; i++) if (deg[i] != 1) rt = i, i = n + 1;
	dep[0] = -1;
	dfs(rt, 0);
	for (int i = 1; i <= n; i++) if (deg[i] == 1) p[i].insert(dep[i]);
	solve(rt, 0);
	cout << p[rt].size() << endl;
	return 0;
}