import sys
sys.setrecursionlimit(2000)
input=sys.stdin.readline
mod=10**9+7
n,c=map(int,input().split())
minc=min(n*6-5,c)
edge=[[]for _ in range(n)]
for _ in range(n-1):
  a,b=map(int,input().split())
  a-=1
  b-=1
  edge[a].append(b)
  edge[b].append(a)
dp=[minc*[1]for _ in range(n)]
def dfs(r,node):
  for mode in edge[node]:
    if r==mode:continue
    dpsub=dfs(node,mode)
    for i in range(minc):
      f=0
      if i>2:f+=dpsub[i-3]
      if i<minc-3:f+=dpsub[i+3]
      dp[node][i]*=f
      dp[node][i]%=mod
  return dp[node]
lastdp=dfs(-1,0)
print((sum(lastdp)+lastdp[minc//2]*(c-minc))%mod)