n,k=map(int,input().split())
if n==k:
  print(1)
  exit()
e=[[] for i in range(n)]
for i in range(n-1):
  a,b=map(int,input().split())
  e[a]+=[b]
  e[b]+=[a]
M=10**9+7
v=[0]*n
w=[[] for i in range(n)]
u=[0]*n
q=[0]
while len(q)>0:
  s=q[-1]
  if v[s]==0:
    v[s]=1
    q+=[t for t in e[s] if v[t]==0]
  else:
    u[s]=1
    w[s]=[0]
    w[s][0]=1
    for t in e[s]:
      if v[t]==0:
        w[t]+=[1]
        nw=[0]*(u[s]+u[t])
        for i in range(u[s]):
          for j in range(u[t]+1):
            nw[i+j]+=w[s][i]*w[t][j]
            nw[i+j]%=M
        w[s]=nw
        u[s]+=u[t]
    v[s]=0
    q.pop()
print(w[0][k])