#include <bits/stdc++.h>
using namespace std;
#define endl '\n'
const int mod=1e9+7;
int binpow(int n,long long k){
int curr=1;
for(;k;k>>=1){
if(k&1){
curr=(1LL*curr*n)%mod;
}
n=(1LL*n*n%mod);
}
return curr;
}
int x=0,y=0,z=0;
const int MAXN=1e5;
vector<vector<int>> graph(MAXN+1);
bool res[MAXN+1];
int sz[MAXN+1];
struct bruh{
int x=0,y=0,z=0;
};
bruh DP[MAXN+1];
void setup(int u,int p){
res[u]=false;
sz[u]=1;
for(int v:graph[u]){
if(v!=p){
setup(v,u);
sz[u]+=sz[v];
if(!res[v]){
res[u]=true;
}
}
}
int child=0,lose=0;
for(int v:graph[u]){
if(v==p){
continue;
}
child++;
if(res[v]){
lose++;
}
}
if(lose==child){
DP[u].y++;
}
else{
DP[u].x++;
}
for(int v:graph[u]){
if(v==p){
continue;
}
if(res[v]){
lose--;
}
if(lose==child-1){
DP[u].x+=(sz[v]-DP[v].x-DP[v].y-DP[v].z);
DP[u].y+=DP[v].z;
DP[u].z+=DP[v].y;
}
else{
DP[u].x+=sz[v];
}
if(res[v]){
lose++;
}
}
}
int x1,y1,z1;
int wcnt=0;
void DFS(int u,int p){
res[u]=false;
sz[u]=1;
for(int v:graph[u]){
sz[u]+=sz[v];
if(!res[v]){
res[u]=true;
}
}
if(res[u]){
wcnt++;
}
vector<int> candi;
DP[u].x=0,DP[u].y=0,DP[u].z=0;
int child=0,lose=0;
for(int v:graph[u]){
child++;
if(res[v]){
lose++;
}
else{
candi.push_back(v);
}
}
if(lose==child){
DP[u].y++;
}
else{
DP[u].x++;
}
int sumx=0,sumy=0,sumz=0;
for(int v:graph[u]){
if(res[v]){
lose--;
}
sumx+=DP[v].x,sumy+=DP[v].y,sumz+=DP[v].z;
if(lose==child-1){
DP[u].x+=(sz[v]-DP[v].x-DP[v].y-DP[v].z);
DP[u].y+=DP[v].z;
DP[u].z+=DP[v].y;
}
else{
DP[u].x+=sz[v];
}
if(res[v]){
lose++;
}
}
x+=DP[u].x,y+=DP[u].y,z+=DP[u].z;
x%=mod,y%=mod,z%=mod;
if(u==1){
x1=DP[u].x,y1=DP[u].y,z1=DP[u].z;
}
for(int i=0;i<graph[u].size();i++){
int v=graph[u][i];
if(v==p){
continue;
}
bruh oriu=DP[u],oriv=DP[v];
bool resu=res[u],resv=res[v];
int szu=sz[u],szv=sz[v];
sz[u]-=sz[v];
if(res[v]){
lose--;
}
child--;
sumx-=DP[v].x,sumy-=DP[v].y,sumz-=DP[v].z;
if(lose==child){
res[u]=false;
}
else{
res[u]=true;
}
DP[u].x=0,DP[u].y=0,DP[u].z=0;
if(lose==child){
DP[u].x=(sz[u]-1-sumx-sumy-sumz);
DP[u].y=sumz+1;
DP[u].z=sumy;
}
else if(lose==child-1){
int vc=-1;
for(int j=0;j<candi.size();j++){
if(candi[j]!=v){
vc=candi[j];
break;
}
}
DP[u].x=sz[u]-sz[vc]+(sz[vc]-DP[vc].x-DP[vc].y-DP[vc].z);
DP[u].y=DP[vc].z;
DP[u].z=DP[vc].y;
}
else{
DP[u].x=sz[u];
}
DFS(v,u);
DP[u]=oriu,DP[v]=oriv;
res[u]=resu,res[v]=resv;
sz[u]=szu,sz[v]=szv;
if(res[v]){
lose++;
}
child++;
sumx+=DP[v].x,sumy+=DP[v].y,sumz+=DP[v].z;
}
}
signed main(){
ios_base::sync_with_stdio(false);
cin.tie(NULL);
long long n,d;
cin >> n >> d;
for(int i=0;i<n-1;i++){
int u,v;
cin >> u >> v;
graph[u].push_back(v),graph[v].push_back(u);
}
if(n==2){
cout << binpow(1LL*n*n%mod,d);
return 0;
}
setup(1,-1);
DFS(1,-1);
for(int i=d-1;i>=0;i--){
if(i==0){
x=x1,y=y1,z=z1;
}
int lcnt=((1LL*binpow(1LL*n*n%mod,d-i-1)*n%mod)-wcnt+mod)%mod;
int nxtw=0;
nxtw+=(1LL*x*binpow(1LL*n*n%mod,d-i-1)%mod*n%mod);
nxtw%=mod;
nxtw+=(1LL*y*lcnt%mod);
nxtw%=mod;
nxtw+=(1LL*z*wcnt%mod);
nxtw%=mod;
wcnt=nxtw;
}
cout << wcnt;
}
