N,K=map(int,raw_input().split())
parent=[-1 for i in range(N)]
child=[[] for i in range(N)]
mod=1000000007
for i in range(N-1):
    a,b=map(int,raw_input().split())
    parent[b]=a
    child[a].append(b)
dp=[[0 for j in range(N+1)] for k in range(N)]
for i in range(N-1,-1,-1):
    if len(child[i])==0:
        dp[i][0]=1
        dp[i][1]=1
        continue
    l=[(0,1)]
    for c in child[i]:
        nl=[]
        for j in range(N+1):
            if dp[c][j]==0:
                break
            for key,val in l:
                nl.append((key+j,val*dp[c][j]%mod))
        l=nl
    s=0
    for key,val in l:
        dp[i][key]+=val
        dp[i][key]%=mod
        s=max(s,key)
    dp[i][s+1]=1
print dp[0][K]