using System;
using System.IO;
using System.Linq;
using System.Text;
using System.Collections.Generic;
using System.Diagnostics;
using System.Numerics;
using Enu = System.Linq.Enumerable;

class Program
{
    static readonly int Mod = (int)1e9 + 7;
    int N, K;
    List<int>[] G;
    int[][] Count;
    int[] Size;

    public void Solve()
    {
        N = Reader.Int();
        K = Reader.Int();
        G = Enu.Range(0, N).Select(i => new List<int>()).ToArray();
        foreach (var e in Reader.IntTable(N - 1))
        {
            G[e[0]].Add(e[1]);
            G[e[1]].Add(e[0]);
        }
        Count = new int[N][];
        Size = new int[N];
        Rec(0, -1);

        Console.WriteLine(Count[0][K]);
    }

    void Rec(int v, int prev)
    {
        if (Count[v] != null) return;
        Count[v] = new int[N + 1];
        Count[v][0] = 1;
        Size[v] = 1;

        foreach (int child in G[v])
            if (child != prev)
            {
                Rec(child, v);
                int[] next = new int[N + 1];
                for (int a = 0; a <= Size[v]; a++)
                    for (int b = 0; b <= Size[child]; b++)
                        next[a + b] = (int)((next[a + b] + (long)Count[v][a] * Count[child][b]) % Mod);
                Count[v] = next;
                Size[v] += Size[child];
            }
        Count[v][Size[v]] = 1;
    }

}


class Entry { static void Main() { new Program().Solve(); } }
class Reader
{
    private static TextReader reader = Console.In;
    private static readonly char[] separator = { ' ' };
    private static readonly StringSplitOptions op = StringSplitOptions.RemoveEmptyEntries;
    private static string[] A = new string[0];
    private static int i;
    private static void Init() { A = new string[0]; }
    public static void Set(TextReader r) { reader = r; Init(); }
    public static void Set(string file) { reader = new StreamReader(file); Init(); }
    public static bool HasNext() { return CheckNext(); }
    public static string String() { return Next(); }
    public static int Int() { return int.Parse(Next()); }
    public static long Long() { return long.Parse(Next()); }
    public static double Double() { return double.Parse(Next()); }
    public static int[] IntLine() { return Array.ConvertAll(Split(Line()), int.Parse); }
    public static int[] IntArray(int N) { return Enu.Range(0, N).Select(i => Int()).ToArray(); }
    public static int[][] IntTable(int H) { return Enu.Range(0, H).Select(i => IntLine()).ToArray(); }
    public static string[] StringArray(int N) { return Enu.Range(0, N).Select(i => Line()).ToArray(); }
    public static string Line() { return reader.ReadLine().Trim(); }
    private static string[] Split(string s) { return s.Split(separator, op); }
    private static string Next() { CheckNext(); return A[i++]; }
    private static bool CheckNext()
    {
        if (i < A.Length) return true;
        string line = reader.ReadLine();
        if (line == null) return false;
        if (line == "") return CheckNext();
        A = Split(line);
        i = 0;
        return true;
    }
}