如题。

#include <bits/stdc++.h>
#include <ext/pb_ds/priority_queue.hpp>
#define int long long
#define pq __gnu_pbds::priority_queue
#define pqg pq<int, vector<int>, greater<int> ()>

using namespace std;

struct ios {
    inline char read() {
        static const int inlen = 1 << 18 | 1;
        static char buf[inlen], *s, *t;
        return (s == t) && (t = (s = buf) + fread(buf, 1, inlen, stdin)), s == t ? -1 : *s++;
    }
    template<typename T> inline ios& operator>> (T &x) {
        static char c11, boo;
        for (c11 = read(), boo = 0; !isdigit(c11); c11 = read()) {
            if (c11 == -1) return *this;
            boo |= c11 == '-';
        }
        for (x = 0; isdigit(c11); c11 = read()) x = x * 10 + (c11 ^ '0');
        boo && (x = -x);
        return *this;
    }
} fin;

struct exios {
    template<typename _CharT, typename _Traits = char_traits<_CharT>>
    struct typ {
        typedef basic_ostream<_CharT, _Traits>& (* end) (basic_ostream<_CharT, _Traits>&);
    };

    template<typename T> friend exios &operator<<(exios &out, T num) {
        if (num < 0) putchar('-'), num = -num;
        if (num >= 10) out << num / 10;
        putchar(num % 10 + '0');
        return out;
    }

    friend exios &operator<<(exios &out, const char * s) { printf("%s", s); return out; }
    friend exios &operator<<(exios &out, string s) { cout << s; return out; }
    friend exios &operator<<(exios &out, typ<char>::end e) { puts(""); return out; }
} fout;

const int maxn = 1e6 + 10;

int n, k, rt;
vector<int> g[maxn];
int deg[maxn];
int dep[maxn];
pq<int, greater<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);
	}
}

void solve(int u, int fa) {
	if (deg[u] == 1) return;
	for (int v : g[u]) {
		if (v == fa) continue;
		solve(v, u);
	}
	pq<int, greater<int>> res = pq<int, greater<int>>(), nw = pq<int, greater<int>>();
	for (int v : g[u]) res.join(p[v]);
	int x, y = res.top();
	while (res.size() >= 2) {
		x = res.top(), res.pop();
		y = res.top();
		if (x + y - dep[u] * 2 > k) nw.push(x);
	}
	nw.push(y);
	p[u] = nw;
}

signed main() {
	fin >> n >> k;
	for (int i = 1, u, v; i < n; i++) {
		fin >> 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].push(dep[i]);
	solve(rt, 0);
	fout << p[rt].size() << endl;
	return 0;
}