#include<bits/stdc++.h>
using namespace std;
const long long inf = (long long) 1e18 + 10;
const int inf1 = (int) 1e9 + 10;
#define int long long
#define dbl long double
#define endl '\n'
#define sc second
#define fr first
#define mp make_pair
#define pb push_back
#define all(x) x.begin(), x.end()
const int maxn = 5e5+10;
const int mod = 1e9+7;
int n, d, dpw[maxn][2], dpl[maxn][2], dpwup[maxn][2], dplup[maxn][2];
vector<int> g[maxn];
int SL, SW, SL0, SW0, SWb, SLb;
void dfs1(int u, int ant) {
dpw[u][1] = SW*dpw[u][0] + SL*(dpw[u][0]+dpl[u][0]); dpw[u][1]%= mod;
dpl[u][1] = SW*dpl[u][0]; dpl[u][1]%= mod;
int qtl = 0;
for(auto v : g[u]) if(v != ant) {
qtl+= dpl[v][0];
dfs1(v,u);
}
for(auto v : g[u]) if(v != ant) {
qtl-= dpl[v][0];
if(qtl) {
dpw[u][1]+= dpw[v][1]+dpl[v][1]; dpw[u][1]%= mod;
dpl[u][1]+= 0;
}
else {
dpw[u][1]+= dpl[v][1]; dpw[u][1]%= mod;
dpl[u][1]+= dpw[v][1]; dpl[u][1]%= mod;
}
qtl+= dpl[v][0];
}
}
map<int,int> wp[maxn];
int solwp(int u, int ant) {
if(wp[u].count(ant)) return wp[u][ant];
wp[u][ant] = 0;
for(auto v : g[u]) if(v != ant) {
if(solwp(v,u) == 0) wp[u][ant] = 1;
}
return wp[u][ant];
}
// void dfs0(int u, int ant) {
// dpw[u][0] = 0;
// for(auto v : g[u]) if(v != ant) {
// dfs0(v,u);
// }
// for(auto v : g[u]) if(v != ant) {
// if(solwp(v,u) == 0) dpw[u][0] = 1;
// }
// dpl[u][0] = 1-dpw[u][0];
// }
void dfs0(int u, int ant) {
dpw[u][0] = 0;
int qtl = 0;
for(auto v : g[u]) if(v != ant) {
dfs0(v,u);
qtl+= dpl[v][0];
}
if(qtl) dpw[u][0] = 1;
dpl[u][0] = 1-dpw[u][0];
}
void dfs0up(int u, int ant) {
int qtl = 0;
for(auto v : g[u]) if(v != ant) {
qtl+= dpl[v][0];
}
for(auto v : g[u]) if(v != ant) {
qtl-= dpl[v][0];
if(qtl || dpwup[u][0]) {
dpwup[v][0] = 0;
dplup[v][0] = 1-dpwup[v][0];
}
else {
dpwup[v][0] = 1;
dplup[v][0] = 1-dpwup[v][0];
}
qtl+= dpl[v][0];
}
for(auto v : g[u]) if(v != ant) {
dfs0up(v,u);
}
}
int dpwc[maxn], dplc[maxn], dpwupc[maxn], dplupc[maxn];
void dfscnt(int u, int ant, int b) {
dpwc[u] = 0;
int qtl = 0;
for(auto v : g[u]) if(v != ant) {
dfscnt(v,u,b);
qtl+= dplc[v];
}
if(qtl || u == b) dpwc[u] = 1;
dplc[u] = 1-dpwc[u];
}
void dfscntup(int u, int ant, int b) {
int qtl = 0;
for(auto v : g[u]) if(v != ant) {
qtl+= dplc[v];
}
for(auto v : g[u]) if(v != ant) {
qtl-= dplc[v];
if((qtl || dpwupc[u]) && v != b) {
dpwupc[v] = 0;
dplupc[v] = 1-dpwupc[v];
}
else {
dpwupc[v] = 1;
dplupc[v] = 1-dpwupc[v];
}
qtl+= dplc[v];
}
for(auto v : g[u]) if(v != ant) {
dfscntup(v,u,b);
}
}
void solve() {
cin >> n >> d;
for(int i = 1; i <= n-1; i++) {
int u,v; cin >> u >> v;
g[u].pb(v);
g[v].pb(u);
}
dfs0(1,0);
dpwup[1][0] = 0;
dplup[1][0] = 1;
dfs0up(1,0);
for(int i = 1; i <= n; i++) {
if(dpw[i][0] || dpwup[i][0]) SW0++;
else SL0++;
}
for(int i = 1; i <= n; i++) {
dfscnt(1,0,i);
if(i == 1) {
dpwupc[1] = 1;
dplupc[1] = 0;
}
else {
dpwupc[1] = 0;
dplupc[1] = 1;
}
dfscntup(1,0,i);
for(int j = 1; j <= n; j++) {
// cout << i << " " << j << " " << (dpwc[j] || dpwupc[j]) << endl;
if(dpwc[j] || dpwupc[j]) SWb++;
else SLb++;
}
}
// cout << SWb << " " << SLb << endl;
// cout << SW0 << " " << SL0 << endl;
SWb%= mod;
SLb%= mod;
SW = SW0;
SL = SL0;
for(int i = 1; i <= d-1; i++) {
int newSW = n*(SW*SW0%mod)%mod + SL*SWb; newSW%= mod;
int newSL = n*(SW*SL0%mod)%mod + SL*SLb; newSL%= mod;
SW = newSW;
SL = newSL;
}
dfs1(1,0);
// cout << SW << " " << SL << endl;
cout << dpw[1][1] << endl;
}
int32_t main() {
ios::sync_with_stdio(false); cin.tie(0);
// freopen("in.in", "r", stdin);
// freopen("out.out", "w", stdout);
int tt = 1;
// cin >> tt;
while(tt--) {
solve();
}
}
