package contest200911;

import java.io.ByteArrayInputStream;
import java.io.IOException;
import java.io.InputStream;
import java.io.PrintWriter;
import java.util.Arrays;
import java.util.InputMismatchException;

// わかんない
public class E {
	InputStream is;
	PrintWriter out;
	String INPUT = "";

	// 5, 4:1/(1-x)
	// 3: 2(x+1/x)
	// 2: 1
	// 1: (2(x+1/x)+1)(x+1/x)

	void solve()
	{
		BarrettInt bi = new BarrettInt(1000000007);
		int n = ni(), C = ni();
		int[] from = new int[n-1];
		int[] to = new int[n-1];
		for(int i = 0;i < n-1;i++){
			from[i] = ni()-1;
			to[i] = ni()-1;
		}
		int[][] g = packU(n, from, to);

		int mod = 1000000007;
		int[][] pars = parents(g, 0);
		int[] par = pars[0], ord = pars[1];

		long[] vals = new long[1002];
		for(int w = 1;w <= 1001;w++) {
			int[][] dp = new int[n][w];
			for (int i = n - 1; i >= 0; i--) {
				int cur = ord[i];
				for (int j = 0; j < w; j++) {
					long t = 1;
					for (int e : g[cur]) {
						if (par[cur] == e) continue;
//						t = t * ((j - 1 >= 0 ? dp[e][j - 1] : 0) + (j + 1 < w ? dp[e][j+1] : 0)) %mod;
						t = bi.modh(t * ((j - 1 >= 0 ? dp[e][j - 1] : 0) + (j + 1 < w ? dp[e][j+1] : 0)));
					}
					dp[cur][j] = (int)t;
				}
			}
			for(int i = 0;i < w;i++){
				vals[w] += dp[0][i];
			}
			vals[w] %= mod;
		}
		for(int i = 2;i < 1002;i++){
			for(int j = 1;j < i;j++){
				vals[i] -= vals[j] * (i-j+1);
				vals[i] %= mod;
			}
			if(vals[i] < 0)vals[i] += mod;
		}
		long ret = 0;
		for(int u = 1;u <= 3;u++){
			int K = (C - u) / 3 + 1;
			for(int i = 1;i <= K && i < 1002;i++){
				ret += vals[i] * (K-i+1);
				ret %= mod;
			}
		}
		out.println(ret);
	}

	public static class BarrettInt {
		public int mod;
		public long M;
		public int h;

		public BarrettInt(int mod)
		{
			this.mod = mod;
			long K = Integer.highestOneBit(mod)<<1;
			h = Long.numberOfTrailingZeros(K)*2;
			M = K*K/mod;
		}

		public long mod(long a)
		{
			long v = M*(a&mask);
			long bl = v&mask;
			v = (v>>>31) + M*(a>>>31);
			bl |= (v&mask)<<31;

			long q = bl>>>h|v>>>31<<62-h;
			long r = a-q*mod;
			return r < mod ? r : r-mod;
		}

		public long modh(long a)
		{
			long r = a-((M*(a&mask)>>>31)+M*(a>>>31)>>>h-31)*mod;
			return r < mod ? r : r-mod;
		}

		public long modl(long a)
		{
			long r = a-(M*a>>>h)*mod;
			return r < mod ? r : r-mod;
		}

		private static final long mask = (1L<<31)-1;
	}


	public static int[][] parents(int[][] g, int root) {
		int n = g.length;
		int[] par = new int[n];
		Arrays.fill(par, -1);

		int[] depth = new int[n];
		depth[0] = 0;

		int[] q = new int[n];
		q[0] = root;
		for (int p = 0, r = 1; p < r; p++) {
			int cur = q[p];
			for (int nex : g[cur]) {
				if (par[cur] != nex) {
					q[r++] = nex;
					par[nex] = cur;
					depth[nex] = depth[cur] + 1;
				}
			}
		}
		return new int[][]{par, q, depth};
	}


	public static int[][] packU(int n, int[] from, int[] to) {
		return packU(n, from, to, from.length);
	}

	public static int[][] packU(int n, int[] from, int[] to, int sup) {
		int[][] g = new int[n][];
		int[] p = new int[n];
		for (int i = 0; i < sup; i++) p[from[i]]++;
		for (int i = 0; i < sup; i++) p[to[i]]++;
		for (int i = 0; i < n; i++) g[i] = new int[p[i]];
		for (int i = 0; i < sup; i++) {
			g[from[i]][--p[from[i]]] = to[i];
			g[to[i]][--p[to[i]]] = from[i];
		}
		return g;
	}


	void run() throws Exception
	{
//		int n = 1000, m = 99999;
//		Random gen = new Random(1);
//		StringBuilder sb = new StringBuilder();
//		sb.append(n + " ");
//		sb.append(1000000000 + " ");
//		for (int i = 0; i < n-1; i++) {
//			sb.append(i+2 + " ");
//			sb.append(i+1 + " ");//gen.nextInt(i+1) + 1 + " ");
//		}
//		INPUT = sb.toString();


		is = INPUT.isEmpty() ? System.in : new ByteArrayInputStream(INPUT.getBytes());
		out = new PrintWriter(System.out);
		
		long s = System.currentTimeMillis();
		solve();
		out.flush();
		if(!INPUT.isEmpty())tr(System.currentTimeMillis()-s+"ms");
//		Thread t = new Thread(null, null, "~", Runtime.getRuntime().maxMemory()){
//			@Override
//			public void run() {
//				long s = System.currentTimeMillis();
//				solve();
//				out.flush();
//				if(!INPUT.isEmpty())tr(System.currentTimeMillis()-s+"ms");
//			}
//		};
//		t.start();
//		t.join();
	}
	
	public static void main(String[] args) throws Exception { new E().run(); }
	
	private byte[] inbuf = new byte[1024];
	public int lenbuf = 0, ptrbuf = 0;
	
	private int readByte()
	{
		if(lenbuf == -1)throw new InputMismatchException();
		if(ptrbuf >= lenbuf){
			ptrbuf = 0;
			try { lenbuf = is.read(inbuf); } catch (IOException e) { throw new InputMismatchException(); }
			if(lenbuf <= 0)return -1;
		}
		return inbuf[ptrbuf++];
	}
	
	private boolean isSpaceChar(int c) { return !(c >= 33 && c <= 126); }
	private int skip() { int b; while((b = readByte()) != -1 && isSpaceChar(b)); return b; }
	
	private double nd() { return Double.parseDouble(ns()); }
	private char nc() { return (char)skip(); }
	
	private String ns()
	{
		int b = skip();
		StringBuilder sb = new StringBuilder();
		while(!(isSpaceChar(b))){ // when nextLine, (isSpaceChar(b) && b != ' ')
			sb.appendCodePoint(b);
			b = readByte();
		}
		return sb.toString();
	}
	
	private char[] ns(int n)
	{
		char[] buf = new char[n];
		int b = skip(), p = 0;
		while(p < n && !(isSpaceChar(b))){
			buf[p++] = (char)b;
			b = readByte();
		}
		return n == p ? buf : Arrays.copyOf(buf, p);
	}
	
	private int[] na(int n)
	{
		int[] a = new int[n];
		for(int i = 0;i < n;i++)a[i] = ni();
		return a;
	}
	
	private long[] nal(int n)
	{
		long[] a = new long[n];
		for(int i = 0;i < n;i++)a[i] = nl();
		return a;
	}
	
	private char[][] nm(int n, int m) {
		char[][] map = new char[n][];
		for(int i = 0;i < n;i++)map[i] = ns(m);
		return map;
	}
	
	private int[][] nmi(int n, int m) {
		int[][] map = new int[n][];
		for(int i = 0;i < n;i++)map[i] = na(m);
		return map;
	}
	
	private int ni() { return (int)nl(); }
	
	private long nl()
	{
		long num = 0;
		int b;
		boolean minus = false;
		while((b = readByte()) != -1 && !((b >= '0' && b <= '9') || b == '-'));
		if(b == '-'){
			minus = true;
			b = readByte();
		}
		
		while(true){
			if(b >= '0' && b <= '9'){
				num = num * 10 + (b - '0');
			}else{
				return minus ? -num : num;
			}
			b = readByte();
		}
	}
	
	private static void tr(Object... o) { System.out.println(Arrays.deepToString(o)); }
}