import scala.io.StdIn.readLine
import scala.collection.mutable.ArrayBuffer

@main
def yuki196(): Unit =
  val MOD = 1000000007
  val Array(n, k) = readLine.split(" ").map(_.toInt)
  val graph = Array.fill(n)(ArrayBuffer[Int]())
  for
    _ <- 1 until n
  do
    val Array(u, v) = readLine.split(" ").map(_.toInt)
    graph(u).append(v)
    graph(v).append(u)
  
  val visited = Array.fill(n)(false)
  val size = Array.fill(n)(0)
  val dp = Array.fill(n)(ArrayBuffer[Int]())

  def dfs(cur: Int): Unit =
    visited(cur) = true
    size(cur) = 1
    var dpCur = ArrayBuffer(1)
    for
      next <- graph(cur)
      if !visited(next)
    do
      dfs(next)
      val dpNext = dp(next)
      val ndp = ArrayBuffer.fill(size(cur) + size(next) - 1)(0)
      for
        i <- dpCur.indices
        j <- dpNext.indices
      do
        ndp(i + j) = (ndp(i + j) + dpCur(i) * dpNext(j)) % MOD
      dpCur = ndp
      size(cur) += size(next) - 1
    dpCur.append(1)
    size(cur) += 1
    dp(cur) = dpCur
  dfs(0)
  println(dp(0)(k))