Language: C++

#include <iostream>
#include <fstream>
#include <cmath>  
#include <cstdlib>
#include <ctime>
#include <algorithm>
#include <numeric>
#include <functional>
#include <string> 
#include <vector>
#include <bitset>
#include <map>
#include <set>
#include <stack>
#include <queue>
#include <deque>

using namespace std;
using ll = long long;

template<class T> using V = vector<T>;
template<class T, class U> using P = pair<T, U>;

#define REP(i,n) for(int i = 0; i < int(n); i++)
#define FOR(i, m, n) for(int i = int(m);i < int(n);i++)
#define ALL(obj) (obj).begin(),(obj).end()

const ll MOD = (ll)1e9 + 7;
const ll HINF = (ll)1e18;
const ll LINF = (ll)1e9;
const long double PI = 3.1415926535897932384626433;

template<class T> void corner(bool flg, T hoge) {
	if (flg) {
		cout << hoge << endl;
		exit(0);
	}
	else return;
}

template <class T, class U>ostream &operator<<(ostream &o, const map<T, U>&obj) {
	o << "{"; for (auto &x : obj) o << " {" << x.first << " : " << x.second << "}" << ","; o << " }"; return o;
}

template <class T>ostream &operator<<(ostream &o, const set<T>&obj) {
	o << "{"; for (auto itr = obj.begin(); itr != obj.end(); ++itr) o << (itr != obj.begin() ? ", " : "") << *itr; o << "}"; return o;
}

template <class T>ostream &operator<<(ostream &o, const vector<T>&obj) {
	o << "{"; for (int i = 0; i < (int)obj.size(); ++i)o << (i > 0 ? ", " : "") << obj[i]; o << "}"; return o;
}

template <class T, class U>ostream &operator<<(ostream &o, const pair<T, U>&obj) {
	o << "{" << obj.first << ", " << obj.second << "}"; return o;
}

template <template <class tmp>  class T, class U> ostream &operator<<(ostream &o, const T<U> &obj) {
	o << "{"; for (auto itr = obj.begin(); itr != obj.end(); ++itr)o << (itr != obj.begin() ? ", " : "") << *itr; o << "}"; return o;
}

void print(void) {
	cout << endl;
}

template <class Head> void print(Head&& head) {
	cout << head;
	print();
}

template <class Head, class... Tail> void print(Head&& head, Tail&&... tail) {
	cout << head << " ";
	print(forward<Tail>(tail)...);
}

void YN(bool flg) {
	cout << ((flg) ? "YES" : "NO") << endl;
}

void Yn(bool flg) {
	cout << ((flg) ? "Yes" : "No") << endl;
}

void yn(bool flg) {
	cout << ((flg) ? "yes" : "no") << endl;
}

const int MAXN = 2000;
int N;ll K;
V<V<ll>> dist(MAXN, V<ll>(MAXN, -1));
V<V<ll>> edge(MAXN);

void bfs(int s){
	queue<int> q;
	q.push(s);
	dist[s][s] = 0;
	while (!q.empty()) {
		int from = q.front();
		q.pop();
		for(auto to: edge[from]){
			if(dist[s][to]==-1){
				q.push(to);
				dist[s][to] = dist[s][from]+1;
			}
		}
	}
	return;
}

template<class T> class Segment_Tree_Range_Maximum_Query {
public:
	int N;
	T ini;
	vector<T> node;

	void init(const vector<T> & ar, const T iini = 0) {
		ini = iini;
		for (N = 1; N < ar.size(); N *= 2);
		node.resize(2 * N - 1, ini);
		for (int i = 0; i<ar.size(); ++i) node[i + N - 1] = ar[i];
		for (int i = N - 2; i >= 0; --i) node[i] = max(node[2 * i + 1], node[2 * i + 2]);
	}

	void update(int idx, const T var) {
		idx += (N - 1);
		node[idx] = var;
		while (idx > 0) {
			idx = (idx - 1) / 2;
			node[idx] = max(node[2 * idx + 1], node[2 * idx + 2]);
		}
	}

	T getvar(const int a, const int b, int k = 0, int l = 0, int r = -1) {
		if (r < 0) r = N;
		if (r <= a || b <= l) return ini;
		if (a <= l && r <= b) return node[k];
		T vl = getvar(a, b, 2 * k + 1, l, (l + r) / 2);
		T vr = getvar(a, b, 2 * k + 2, (l + r) / 2, r);
		return max(vl, vr);
	}
};

template<class T> class Segment_Tree_Range_Sum_Query {
public:
	int N;
	T ini;
	vector<T> node;
	
	void init(const vector<T> & ar, const T iini = 0){
		ini = iini;
		for (N = 1; N < ar.size(); N *= 2);
		node.resize(2 * N - 1, ini);
		for (int i = 0; i<ar.size(); ++i) node[i + N - 1] = ar[i];
		for (int i = N - 2; i >= 0; --i) node[i] = node[2 * i + 1] + node[2 * i + 2];
	}

	void update(int idx, const T var) {
		idx += (N - 1);
		node[idx] += var;
		while (idx > 0) {
			idx = (idx - 1) / 2;
			node[idx] = node[2 * idx + 1] + node[2 * idx + 2];
		}
	}

	T getvar(const int a, const int b, int k = 0, int l = 0, int r = -1) {
		if (r < 0) r = N;
		if (r <= a || b <= l) return ini;
		if (a <= l && r <= b) return node[k];
		T vl = getvar(a, b, 2 * k + 1, l, (l + r) / 2);
		T vr = getvar(a, b, 2 * k + 2, (l + r) / 2, r);
		return vl + vr;
	}
};

int main() {
	cin >> N >> K;
	REP(i,N-1){
		int A, B; 
		scanf_s("%d%d", &A, &B);
		A--; B--;
		edge[A].push_back(B);
		edge[B].push_back(A);
	}
	REP(i, N) bfs(i);
	V<Segment_Tree_Range_Sum_Query<ll>> RSQ(N);
	V<Segment_Tree_Range_Maximum_Query<ll>> RMQ(N);

	REP(i,N){
		RSQ[i].init(dist[i]);
		RMQ[i].init(dist[i]);
	}

	Segment_Tree_Range_Maximum_Query<P<P<ll, ll>, int>> Seg;
	V<P<P<ll, ll>, int>> a(N);
	REP(i, N) a[i] = { { RMQ[i].getvar(0,N), RSQ[i].getvar(0, N) },i };
	Seg.init(a, { {0,0},0 });
	
	REP(i, N) {
		P<P<ll, ll>, int> p = Seg.getvar(0, N);
		ll maxi = p.first.first;
		ll sum = p.first.second;
		int idx = p.second;

		if (maxi <= K) {
			cout << i << endl;
			return 0;
		}

		REP(j, N) {
			RSQ[idx].update(j, -dist[idx][j]);
			RMQ[idx].update(j, 0);
			RSQ[j].update(idx, -dist[j][idx]);
			RMQ[j].update(idx, 0);
			Seg.update(j, { { RMQ[j].getvar(0,N), RSQ[j].getvar(0, N) },j });
		}
	}

	return 0;
}



Compiler output:
a.cc: In function 'int main()':
a.cc:179:17: error: 'scanf_s' was not declared in this scope; did you mean 'scanf'?
  179 |                 scanf_s("%d%d", &A, &B);
      |                 ^~~~~~~
      |                 scanf