Problem:
Score : 600 points

Problem Statement

Given an undirected tree, let the distance between vertices u and v be the
number of edges on the simple path from u to v.
The diameter of a tree is the maximum among the distances between any two vertices.
We will call a tree good if and only if its diameter is at most K.

You are given an undirected tree with N vertices numbered 1 through N.
For each i (1≦i≦N-1), there is an edge connecting vertices A_i and B_i.

You want to remove zero or more vertices from the tree, so that the resulting tree is good.
When a vertex is removed, all incident edges will also be removed.
The resulting graph must be connected.

Find the minimum number of vertices that you need to remove in order to produce a good tree.

Constraints

2≦N≦2000

1≦K≦N-1

1≦A_i≦N, 1≦B_i≦N

The graph defined by A_i and B_i is a tree.

Input

The input is given from Standard Input in the following format:

N K
A_1 B_1
A_2 B_2
:
A_{N-1} B_{N-1}

Output

Print the minimum number of vertices that you need to remove in order to produce a good tree.

Sample Input 1

6 2
1 2
3 2
4 2
1 6
5 6

Sample Output 1

2

The tree is shown below. Removing vertices 5 and 6 will result in a good tree with the diameter of 2.

Sample Input 2

6 5
1 2
3 2
4 2
1 6
5 6

Sample Output 2

0

Since the given tree is already good, you do not need to remove any vertex.

Solution in C# (Mono 4.6.2.0) (judge verdict: Accepted)

using System;
using System.Collections;
using System.Collections.Generic;
using System.IO;
using System.Linq;
using System.Numerics;
using System.Text;
using System.Threading.Tasks;

namespace AtCorder
{
	public class Program
	{
		static void Main(string[] args)
		{
			var cin = new Scanner2();
			int n = cin.Int();
			int k = cin.Int();

			var ab = new Tuple<int, int>[n - 1];
			for (int i = 0; i < n - 1; i++) {
				int a = cin.Int() - 1;
				int b = cin.Int() - 1;
				ab[i] = new Tuple<int, int>(a, b);
			}

			var to = new List<int>[n];
			for (int i = 0; i < n; i++) {
				to[i] = new List<int>();
			}

			for (int i = 0; i < n - 1; i++) {
				int a = ab[i].Item1;
				int b = ab[i].Item2;
				to[a].Add(b);
				to[b].Add(a);
			}

			int ans = int.MaxValue;
			if (k % 2 == 0) {
				int d = k / 2;
				for (int i = 0; i < n; i++) {
					var ds = GetDistance(n, i, to);
					int count = ds.Where(x => x > d).Count();
					ans = Math.Min(ans, count);
				}
			} else {
				int d = (k - 1) / 2;
				for (int i = 0; i < n - 1; i++) {
					var ds = GetDistance(n, ab[i].Item1, ab[i].Item2, to);
					int count = ds.Where(x => x > d).Count();
					ans = Math.Min(ans, count);
				}
			}

			Console.WriteLine(ans);

		}

		private static int[] GetDistance(int n, int s, List<int>[] to)
		{
			var d = new int[n];
			for (int i = 0; i < n; i++) {
				d[i] = -1;
			}

			d[s] = 0;
			var q = new Queue<int>();
			q.Enqueue(s);
			while (q.Count > 0) {
				int cur = q.Dequeue();
				foreach (int next in to[cur]) {
					if (d[next] == -1) {
						d[next] = d[cur] + 1;
						q.Enqueue(next);
					}
				}
			}

			return d;
		}

		private static int[] GetDistance(int n, int a, int b, List<int>[] to)
		{
			var d = new int[n];
			for (int i = 0; i < n; i++) {
				d[i] = -1;
			}

			d[a] = 0;
			d[b] = 0;
			var q = new Queue<int>();
			q.Enqueue(a);
			q.Enqueue(b);
			while (q.Count > 0) {
				int cur = q.Dequeue();
				foreach (int next in to[cur]) {
					if (d[next] == -1) {
						d[next] = d[cur] + 1;
						q.Enqueue(next);
					}
				}
			}

			return d;
		}
	}

	public class Scanner2
	{
		private readonly char[] delimiter_ = new char[] { ' ' };
		private readonly string filePath_;
		private readonly Func<string> reader_;
		private string[] buf_;
		private int index_;

		public Scanner2(string file = "")
		{
			if (string.IsNullOrWhiteSpace(file)) {
				reader_ = Console.ReadLine;
			} else {
				filePath_ = file;
				var fs = new StreamReader(file);
				reader_ = fs.ReadLine;
			}
			buf_ = new string[0];
			index_ = 0;
		}

		public string Next()
		{
			if (index_ < buf_.Length) {
				return buf_[index_++];
			}

			string st = reader_();
			while (st == "") {
				st = reader_();
			}

			buf_ = st.Split(delimiter_, StringSplitOptions.RemoveEmptyEntries);
			if (buf_.Length == 0) {
				return Next();
			}

			index_ = 0;
			return buf_[index_++];
		}

		public int Int() => int.Parse(Next());
		public long Long() => long.Parse(Next());
		public double Double() => double.Parse(Next());

		public int[] ArrayInt(int N, int add = 0)
		{
			int[] Array = new int[N];
			for (int i = 0; i < N; i++) {
				Array[i] = Int() + add;
			}
			return Array;
		}

		public long[] ArrayLong(int N, long add = 0)
		{
			long[] Array = new long[N];
			for (int i = 0; i < N; i++) {
				Array[i] = Long() + add;
			}
			return Array;
		}

		public double[] ArrayDouble(int N, double add = 0)
		{
			double[] Array = new double[N];
			for (int i = 0; i < N; i++) {
				Array[i] = Double() + add;
			}
			return Array;
		}

		public void Save(string text)
		{
			if (string.IsNullOrWhiteSpace(filePath_)) {
				return;
			}

			File.WriteAllText(filePath_ + "_output.txt", text);
		}
	}
}