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)

smx = min(3*(n-1), c-1)
mx = min(smx+3*(n-1), c-1)

dp = [[0]*(mx+1) for _ in range(n)]
def dfs():
    stack = [(0, 0, True)]
    while stack:
        u, par, first = stack.pop()
        if first:
            stack.append((u, par, False))
            for v in to[u]:
                if v == par: continue
                stack.append((v, u, True))
        else:
            if u != 0 and len(to[u]) == 1:
                for j in range(mx+1):
                    dp[u][j] = 1
            else:
                for j in range(mx+1):
                    val = 1
                    for v in to[u]:
                        if v == par: continue
                        cur = 0
                        if j-3 >= 0: cur += dp[v][j-3]
                        if j+3 <= mx: cur += dp[v][j+3]
                        val = val*cur%md
                    dp[u][j] = val

dfs()

ans = 0
for i in range(smx+1):
    j = c-1-i
    if i > j: break
    if i == j:
        ans += dp[0][i]
        break
    ans += dp[0][i]*2

if c-6*n+7 > 0:
    ans += dp[0][smx]*(c-6*n+7)

ans %= md
print(ans)