# いわゆる「二乗の木DP」

N,K=map(int,input().split())
E=[[] for i in range(N)]

mod=10**9+7

for i in range(N-1):
    a,b=map(int,input().split())
    E[a].append(b)
    E[b].append(a)

ROOT=0
QUE=[ROOT] 
Parent=[-1]*N
Parent[ROOT]=N # ROOTの親を定めておく.
Child=[[] for i in range(N)]
TOP_SORT=[] # トポロジカルソート

while QUE: # トポロジカルソートと同時に親を見つける
    x=QUE.pop()
    TOP_SORT.append(x)
    for to in E[x]:
        if Parent[to]==-1:
            Parent[to]=x
            Child[x].append(to)
            QUE.append(to)

DP=[[] for i in range(N)]

for x in TOP_SORT[::-1]:
    if len(Child[x])==0:
        DP[x]=[1,1]
        continue

    NDP=[1]
    
    for to in Child[x]:
        XDP=[0]*(len(DP[to])+len(NDP)-1)
        for i in range(len(DP[to])):
            for j in range(len(NDP)):
                XDP[i+j]+=DP[to][i]*NDP[j]%mod
                XDP[i+j]%=mod
        NDP=XDP

    NDP.append(1)

    DP[x]=NDP

print(DP[0][K]%mod)