import numpy as n
N,K=map(int,input().split())
c=[[] for _ in range(N)]
for i in range(N-1):
	a,b=map(int,input().split())
	c[a].append(b)
def f(x):
	p,v=1,n.array([1],dtype="O")
	for i in c[x]:
		q,y=f(i)
		p+=q
		v=n.convolve(v,y)%(1e9+7)
	return p,n.append(v,1)

print(int(f(0)[1][K]))