import sys
sys.setrecursionlimit(2000)
input = sys.stdin.readline

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
      dpsub = dfs(node, mode)
      dp[node][0] = (dp[node][0] * dpsub[1]) % mod
      dp[node][minc - 1] = (dp[node][minc - 1] * dpsub[minc - 2]) % mod
      for i in range(1, minc - 1):
        dp[node][i] = (dp[node][i] * (dpsub[i - 1]+dpsub[i + 1])) % mod
    return dp[node]

  lastdp = dfs(-1, 0)

  if minc == x:
    return sum(lastdp)
  else:
    return sum(lastdp) + (x - minc) * lastdp[minc // 2]


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)

c1 = (c + 2) // 3
c2 = (c + 1) // 3
c3 = c // 3

ans1 = solve(c1)

if c1 == c2:
  ans2 = ans1
elif c2 == 1:
  ans2 = 0
else:
  ans2 = solve(c2)
 
if c2 == c3:
  ans3 = ans2
elif c3 == 1:
  ans3 = 0
else:
  ans3 = solve(c3)


print((ans1 + ans2 + ans3) % mod)