import java.io.IOException;
import java.io.InputStream;
import java.util.ArrayList;
import java.util.Arrays;
import java.util.NoSuchElementException;

public class Main {

	public static void main(String[] args) {
		new Main().run();
	}
	
	long pow(long a,long n) {
		long ret=1;
		for (;n>0;n>>=1,a=a*a%MOD) if (n%2==1) ret=ret*a%MOD;
		return ret;
	}
	
	void dfs(int cur,int par,ArrayList<Integer>[] g,int C,int max) {
		for (int curc=1;curc<=max;++curc) {
			dp[cur][curc]=1;
		}
		for (int dst:g[cur]) {
			if (dst==par) continue;
			dfs(dst,cur,g,C,max);
			for (int curc=1;curc<dp[cur].length;++curc) {
				long sum=0;
				for (int d=-3;d<=3;d+=6) {
					int nextc=curc+d;
					if (nextc<1||nextc>C) continue;
					sum+=dp[dst][nextc];
					sum%=MOD;
				}
				dp[cur][curc]*=sum;
				dp[cur][curc]%=MOD;
			}
		}
	}
	
	final long MOD=(long)1e9+7;
	int MAXN=3000;
	long[][] dp=new long[MAXN][3*MAXN+1];
	
	void run() {
		FastScanner sc=new FastScanner();
		int N=sc.nextInt();
		int C=sc.nextInt();
		int[] a=new int[N-1];
		int[] b=new int[N-1];
		ArrayList<Integer>[] g=new ArrayList[N];
		for (int i=0;i<N;++i) g[i]=new ArrayList<>();
		for (int i=0;i<N-1;++i) {
			a[i]=sc.nextInt()-1;
			b[i]=sc.nextInt()-1;
			g[a[i]].add(b[i]);
			g[b[i]].add(a[i]);
		}
		
		if (C<=6000) {
			dfs(0,-1,g,C,C);
			long ans=0;
			for (long v:dp[0]) {
				ans=(ans+v)%MOD;
			}
			System.out.println(ans);
		} else {
			dfs(0,-1,g,C,3000);
			long s=0;
			for (long v:dp[0]) {
				s=(s+v)%MOD;
			}
			long ans=2*s-(C-6000)*pow(3,N-1)%MOD;
			ans=(ans%MOD+MOD)%MOD;
			System.out.println(ans);
		}
	}
	
	void tr(Object...objects) {System.out.println(Arrays.deepToString(objects));}
	
	
	class FastScanner {
	    private final InputStream in = System.in;
	    private final byte[] buffer = new byte[1024];
	    private int ptr = 0;
	    private int buflen = 0;
	    private boolean hasNextByte() {
	        if (ptr < buflen) {
	            return true;
	        }else{
	            ptr = 0;
	            try {
	                buflen = in.read(buffer);
	            } catch (IOException e) {
	                e.printStackTrace();
	            }
	            if (buflen <= 0) {
	                return false;
	            }
	        }
	        return true;
	    }
	    private int readByte() { if (hasNextByte()) return buffer[ptr++]; else return -1;}
	    private boolean isPrintableChar(int c) { return 33 <= c && c <= 126;}
	    private void skipUnprintable() { while(hasNextByte() && !isPrintableChar(buffer[ptr])) ptr++;}
	    public boolean hasNext() { skipUnprintable(); return hasNextByte();}
	    public String next() {
	        if (!hasNext()) throw new NoSuchElementException();
	        StringBuilder sb = new StringBuilder();
	        int b = readByte();
	        while(isPrintableChar(b)) {
	            sb.appendCodePoint(b);
	            b = readByte();
	        }
	        return sb.toString();
	    }
	    public long nextLong() {
	        if (!hasNext()) throw new NoSuchElementException();
	        long n = 0;
	        boolean minus = false;
	        int b = readByte();
	        if (b == '-') {
	            minus = true;
	            b = readByte();
	        }
	        if (b < '0' || '9' < b) {
	            throw new NumberFormatException();
	        }
	        while(true){
	            if ('0' <= b && b <= '9') {
	                n *= 10;
	                n += b - '0';
	            }else if(b == -1 || !isPrintableChar(b)){
	                return minus ? -n : n;
	            }else{
	                throw new NumberFormatException();
	            }
	            b = readByte();
	        }
	    }
	    public int nextInt() {
	    	return (int)nextLong();
	    }
	}
}