#include <iostream>
#include <bits/stdc++.h>
#define ll long long
using namespace std;
const ll maxn = 1e5+5, INF = 4e18+9, mod = 1e9+7;
vector<ll> fact, invfact;
ll binpow(ll a, ll b){
ll res = 1;
while(b > 0){
if(b&1){
res*=a;
res%=mod;
}
a*=a;
a%=mod;
b/=2;
}
return res;
}
ll modinv(ll x){
return binpow(x, mod-2);
}
void calcfact(int n){
fact.resize(n+1, 0);
invfact.resize(n+1, 0);
fact[0] = 1;
for(int i = 1; i <= n; i++){
fact[i] = fact[i-1]*i;
fact[i]%=mod;
}
invfact[n] = modinv(fact[n]);
for(int i = n-1; i >= 0; i--){
invfact[i] = invfact[i+1]*(i+1)%mod;
}
}
ll nCk(ll n, ll k){
if(k < 0 || k > n) return 0;
return fact[n]*(invfact[k]*invfact[n-k]%mod)%mod;
}
ll distributing_apples(ll n, ll m){ // number of people, apple
return nCk(m+n-1, n-1);
}
void solve(){
ll n, D;
cin >> n >> D;
vector<vector<int>> adj(n+1);
for(int i = 1; i < n; i++){
int u, v;
cin >> u >> v;
adj[u].push_back(v);
adj[v].push_back(u);
}
vector<int> f(n+1), g(n+1);
{
auto dfs = [&](auto dfs, int u, int p) -> void{
f[u] = 0;
g[u] = 0;
int onlyv = 0, vv = -1;
for(int v : adj[u]){
if(v == p) continue;
dfs(dfs, v, u);
if(f[v] == 0) f[u] = 1;
if(f[v] == 0){
vv = v;
onlyv++;
}
}
if(f[u] == 1){
if(onlyv == 1){
for(int v : adj[u]){
if(v == vv){
g[u] += g[v];
}
}
}
}else{
for(int v : adj[u]){
if(f[v] == 1){
g[u] += g[v];
}
}
g[u]++;
}
};
dfs(dfs, 1, 0);
}
{
vector<int> t(n+1), tg(n+1);
auto dfs = [&](auto dfs, int u, int p) -> void{
t[u] = f[u];
tg[u] = g[u];
int m = adj[u].size();
vector<int> pf(m+1, 0), sf(m+2, 0);
vector<int> state(m+1);
vector<int> pfg(m+1, 0), sfg(m+2, 0);
vector<int> sg(m+1, 0);
int it = 0;
for(int v : adj[u]){
it++;
state[it] = (f[v] == 0);
if(f[v] != f[u]) sg[it] = g[v];
}
for(int i = 1; i <= m; i++){
pf[i] = pf[i-1] + state[i];
pfg[i] = pfg[i-1] + sg[i];
}
for(int i = m; i >= 1; i--){
sf[i] = sf[i+1] + state[i];
sfg[i] = sfg[i+1] + sg[i];
}
it = 0;
for(int v : adj[u]){
it++;
if(v == p) continue;
int rb = f[u], rbg = g[u];
f[u] = (pf[it-1] + sf[it+1] > 0);
if(f[u] == 1){
if(pf[it-1] + sf[it+1] == 1){
g[u] = pfg[it-1] + sfg[it+1];
}else{
g[u] = 0;
}
}else{
g[u] = pfg[it-1] + sfg[it+1];
g[u]++;
}
if((f[v] | (f[u] == 0)) == 1){
if(f[v] == 0 && f[u] == 0){
g[v] += g[u];
}else if(f[v] == 1 && f[u] == 0){
g[v] = 0;
}
}else{
g[v] += g[u];
}
f[v] |= (f[u] == 0);
dfs(dfs, v, u);
f[u] = rb;
g[u] = rbg;
}
};
dfs(dfs, 1, 0);
f = t;
g = tg;
}
ll WC = 0, LC = 0, W = 0, L = 0;
for(int i = 1; i <= n; i++){
if(f[i] == 1){
WC += g[i];
W ++;
}else{
LC += g[i];
L ++;
}
}
WC %= mod;
LC %= mod;
ll E = (WC - LC + mod)%mod;
ll LD = L * (binpow(n, 2*D) - binpow(E, D) + mod) % mod * modinv((binpow(n, 2*D)%mod + mod - E)%mod) % mod;
ll ans;
if(f[1] == 0){
ans = LD * g[1] % mod;
}else{
ans = binpow(n, 2*D) - LD * g[1] % mod + mod;
ans %= mod;
}
//cout << binpow(4, D) << "\n";
cout << ans;
}
int main(){
ios_base::sync_with_stdio(false);
cin.tie(NULL);
solve();
}
