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 Java7 (OpenJDK 1.7.0) (judge verdict: Runtime Error)

import java.util.ArrayList;
import java.util.HashMap;
import java.util.List;
import java.util.Map;
import java.util.Scanner;
 
public class Main
{
    private static class Edge
    {
        Vertex from, to;
        long fromDepth, toDepth;
        long fromCost, toCost;
 
        public Edge(Vertex from, Vertex to)
        {
            this.from = from;
            this.to = to;
            this.from.edges.add(this);
            this.to.edges.add(this);
        }
 
        void setFrom(Vertex newFrom)
        {
            // Swap if needed
            if (newFrom == to)
            {
                Vertex tmp = this.from;
                this.from = to;
                to = tmp;
            }
            else if (newFrom != from)
                throw new RuntimeException("Abracadabra froms do not agree");
        }
    }
 
    private static class GraphTraversalData
    {
        final long maxDepthFromThisVertex, maxCostFromThisVertex;
 
        GraphTraversalData(long maxDepthFromThisVertex, long maxCostFromThisVertex)
        {
            this.maxDepthFromThisVertex = maxDepthFromThisVertex;
            this.maxCostFromThisVertex = maxCostFromThisVertex;
        }
    }
 
    private static class Vertex
    {
        final long vindex;
        List<Edge> edges = new ArrayList<Edge>();
 
        Vertex(long vindex)
        {
            this.vindex = vindex;
        }
 
        public String toString()
        {
            return Long.toString(vindex);
        }
 
        public void populateData()
        {
            // First pass to figure out the out/to direction
            for (Edge edge : edges)
            {
                edge.setFrom(this);
                edge.to.traverseGraphAndSetToData(edge);
            }
 
            // Second pass to figure out the in/from direction
            traverseGraphAndSetFromData(null);
        }
 
        void traverseGraphAndSetFromData(Edge arrivedVia)
        {
            long totalCostViaOtherEdges, maxDViaOtherEdges;
 
            for (Edge edge : edges)
            {
                if (edge != arrivedVia)
                {
                    totalCostViaOtherEdges = 0;
                    maxDViaOtherEdges = 0;
                    for (Edge tmpEdge : edges)
                    {
                        if (tmpEdge != edge && tmpEdge != arrivedVia)
                        {
                            totalCostViaOtherEdges += tmpEdge.toCost;
                            maxDViaOtherEdges = Math.max(maxDViaOtherEdges, tmpEdge.toDepth);
                        }
                    }
 
                    if (arrivedVia != null)
                    {
                        totalCostViaOtherEdges += arrivedVia.fromCost;
                        maxDViaOtherEdges = Math.max(maxDViaOtherEdges, arrivedVia.fromDepth);
                    }
 
                    edge.fromCost = totalCostViaOtherEdges + 1;
                    edge.fromDepth = maxDViaOtherEdges + 1;
 
                    edge.to.traverseGraphAndSetFromData(edge);
                }
            }
        }
 
        private GraphTraversalData traverseGraphAndSetToData(Edge arrivedVia)
        {
            long maxDepthFromThisVertex = 0;
            long maxCostFromThisVertex = 1;
            for (Edge edge : edges)
            {
                if (edge != arrivedVia)
                {
                    edge.setFrom(this);
                    GraphTraversalData graphTraversalData = edge.to.traverseGraphAndSetToData(edge);
                    maxCostFromThisVertex += graphTraversalData.maxCostFromThisVertex;
                    maxDepthFromThisVertex = Math.max(maxDepthFromThisVertex, graphTraversalData.maxDepthFromThisVertex);
                }
            }
 
            arrivedVia.toDepth = maxDepthFromThisVertex + 1;
            arrivedVia.toCost = maxCostFromThisVertex;
            return new GraphTraversalData(arrivedVia.toDepth, maxCostFromThisVertex);
        }
    }
 
    public static void main(String[] args)
    {
      Scanner sc = new Scanner(System.in);
      int n = sc.nextInt();
      int d = sc.nextInt();
      String sampleInput = n + " " +d +"\n";
      for (int i = 0; i<n-1;i++){
        String a = sc.next();
        String b= sc.next();
        sampleInput = sampleInput + a+" "+b+"\n";
      }
        solve(sampleInput, false);
    }
 
    private static int solve(String input, boolean testMode)
    {
        String[] lines = input.split("\n");
        String[] NandK = lines[0].split(" ");
        //System.out.println("N is " + NandK[0] + ", K is " + NandK[1]);
 
        long N = Long.parseLong(NandK[0]);
        long K = Long.parseLong(NandK[1]);
 
        Map<Long, Vertex> vertices = new HashMap<Long, Vertex>();
        List<Edge> edges = new ArrayList<Edge>();
        for (int i = 1; i < N; i++)
        {
            String[] edgeInfo = lines[i].split(" ");
            long fromVindex = Long.parseLong(edgeInfo[0]);
            long toVindex = Long.parseLong(edgeInfo[1]);
 
            if (!vertices.containsKey(fromVindex))
                vertices.put(fromVindex, new Vertex(fromVindex));
 
            if (!vertices.containsKey(toVindex))
                vertices.put(toVindex, new Vertex(toVindex));
 
            Vertex fromVertex = vertices.get(fromVindex);
            Vertex toVertex = vertices.get(toVindex);
 
            edges.add(new Edge(fromVertex, toVertex));
        }
 
        if (testMode)
        {
            for (Vertex vertex : vertices.values())
            {
                vertex.populateData();
 
                long result = solveForK(edges, K);
                System.out.println("Result is " + result);
            }
        }
        else
        {
            Vertex vertex = vertices.values().iterator().next();
            vertex.populateData();
 
            long result = solveForK(edges, K);
            if (K < 500)
                throw new RuntimeException("test");
            System.out.println(result);
            
        }
 
        return 0;
    }
 
    static long solveForK(List<Edge> edges, long K)
    {
        long minRemovedVertices = Long.MAX_VALUE;
        for (Edge edge : edges)
        {
            if (edge.fromDepth <= K)
                minRemovedVertices = Math.min(minRemovedVertices, edge.toCost - 1);
            if (edge.toDepth <= K)
                minRemovedVertices = Math.min(minRemovedVertices, edge.fromCost - 1);
        }
 
        return minRemovedVertices == Long.MAX_VALUE ? 0 : minRemovedVertices;
    }
}