import sys
sys.setrecursionlimit(4000)
def f(v,p,E):
  if v and len(E[v])==1: return [1,1]
  L=[1]
  for c in E[v]:
    if c==p: continue
    L=m(L,f(c,v,E))
  L.append(1)
  return L
def m(L1, L2):
  M=10**9+7
  L=[0]*(len(L1)+len(L2)-1)
  for i,a in enumerate(L1):
    for j,b in enumerate(L2): L[i+j]+=a*b
  return [v%M for v in L]
N,K=map(int,input().split())
E=[[] for i in range(N)]
for i in range(N-1):
  a,b=map(int,input().split())
  E[a].append(b)
  E[b].append(a)
L=f(0,-1,E)
L.append(1)
print(L[K])