mod=10**9+7
n,c=map(int,input().split())
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)
def solve(x):
  minc=min(2*n-1,x)
  dp=[minc*[1]for _ in range(n)]
  def dfs(r,node):
    for mode in edge[node]:
      if mode==r:continue
      dp1=dfs(node,mode)
      dp[node][0]=(dp[node][0]*dp1[1])%mod
      dp[node][-1]=(dp[node][-1]*dp1[-2])%mod
      for i in range(1,minc-1):
        dp[node][i]=(dp[node][i]*(dp1[i-1]+dp1[i+1]))%mod
    return dp[node]
  dfs(-1,0)
  if minc==x:return sum(dp[0])
  else:return sum(dp[0])+(x-minc)*dp[0][minc//2]
ans1=solve((c+2)//3) if c>=4 else 0
ans2=solve((c+1)//3) if c>=5 else 0
ans3=solve(c//3) if c>=6 else 0
print(ans1+ans2+ans3)