import sys

# sys.setrecursionlimit(200005)
int1 = lambda x: int(x)-1
p2D = lambda x: print(*x, sep="\n")
def II(): return int(sys.stdin.readline())
def LI(): return list(map(int, sys.stdin.readline().split()))
def LI1(): return list(map(int1, sys.stdin.readline().split()))
def LLI(rows_number): return [LI() for _ in range(rows_number)]
def LLI1(rows_number): return [LI1() for _ in range(rows_number)]
def SI(): return sys.stdin.readline().rstrip()
# dij = [(0, 1), (-1, 0), (0, -1), (1, 0)]
dij = [(0, 1), (-1, 0), (0, -1), (1, 0), (1, 1), (1, -1), (-1, 1), (-1, -1)]
inf = 10**16
# md = 998244353
md = 10**9+7

n, c = LI()
to = [[] for _ in range(n)]
for _ in range(n-1):
    u, v = LI1()
    to[u].append(v)
    to[v].append(u)

maxu = min(3*n-3, c)
maxj = min(6*n-6, c)

dp = [[1]*(maxj+1) for _ in range(n)]

dfsorder = []
parent = [-1]*n
stack = [0]
while stack:
    u = stack.pop()
    dfsorder.append(u)
    for v in to[u]:
        if v == parent[u]: continue
        parent[v] = u
        stack.append(v)
# print(dfsorder)

for u in dfsorder[::-1]:
    for j in range(1, maxj+1):
        for v in to[u]:
            if v == parent[u]: continue
            cur = 0
            if j+3 <= maxj: cur += dp[v][j+3]
            if j-3 > 0: cur += dp[v][j-3]
            dp[u][j] = dp[u][j]*cur%md

ans = sum(dp[0][1:maxu+1])%md
suf = min(c-maxu, maxu)
ans += sum(dp[0][1:1+suf])
ans += (c-maxu-suf)*pow(2, n-1, md)%md
print(ans%md)