#include<bits/stdc++.h>
using namespace std;
mt19937 rng(chrono::steady_clock::now().time_since_epoch().count());
typedef long long ll;
#define int ll
typedef unsigned long long ull;
typedef long double ld;
typedef pair<int, int> pii;
typedef pair<ll, ll> pll;
#define pb push_back
#define all(x) x.begin(), x.end()
#define sz(x) (int)x.size()
#define mispertion ios_base::sync_with_stdio(0), cin.tie(0), cout.tie(0)
#define F first
#define S second
#define getlast(s) (*s.rbegin())
#define debg cout << "OK\n"
const ld PI = 3.1415926535;
const int N = 1e6 + 6;
const int M = 7e6 + 1;
const int mod = 1e9+7;
const int infi = INT_MAX;
const ll infl = LLONG_MAX;
const int P = 31;
int mult(int a, int b) {
return a * 1LL * b % mod;
}
int sum(int a, int b) {
if (a + b < 0)
return a + b + mod;
if (a + b >= mod)
return a + b - mod;
return a + b;
}
ll binpow(ll a, ll n) {
if (n == 0)
return 1;
if (n % 2 == 1) {
return binpow(a, n - 1) * a % mod;
} else {
ll b = binpow(a, n / 2);
return b * b % mod;
}
}
int n, d, dp[N], ndp[N], cd[N], cu[N], C[N], st[N], ddp[N];
vector<int> g[N];
void dfs(int v, int p){
int cnt0 = 0;
for(auto u : g[v]){
if(u == p) continue;
dfs(u, v);
if(dp[u] == 0)
cnt0++;
}
dp[v] = (cnt0 != 0);
if(dp[v] == 0){
cd[v] = 1;
for(auto u : g[v]){
if(u == p) continue;
cd[v] += cd[u];
}
}else{
cd[v] = 0;
if(cnt0 == 1){
for(auto u : g[v]){
if(u == p) continue;
if(dp[u] == 0)
cd[v] += cd[u];
}
}
}
}
void dfs1(int v, int p){
int cnt0 = (ndp[v] == 0), sm0 = 0, sm = cu[v];
if(ndp[v] == 0){
sm0 += cu[v];
}
for(auto u : g[v]){
if(u == p) continue;
cnt0 += (dp[u] == 0);
sm += cd[u];
if(dp[u] == 0)
sm0 += cd[u];
}
for(auto u : g[v]){
if(u == p) continue;
cnt0 -= (dp[u] == 0);
sm -= cd[u];
if(dp[u] == 0)
sm0 -= cd[u];
ndp[u] = (cnt0 != 0);
if(ndp[u] == 0){
cu[u] = 1 + sm;
}else{
if(cnt0 == 1)
cu[u] = sm0;
}
dfs1(u, v);
cnt0 += (dp[u] == 0);
sm += cd[u];
if(dp[u] == 0)
sm0 += cd[u];
}
}
void dfs2(int v, int p){
int cnt0 = (ndp[v] == 0);
for(auto u : g[v]){
if(u == p) continue;
cnt0 += (dp[u] == 0);
}
st[v] = (cnt0 != 0);
if(st[v] == 0){
C[v] = 1 + cu[v];
for(auto u : g[v]){
if(u == p) continue;
C[v] += cd[u];
}
}else{
if(cnt0 == 1){
if(ndp[v] == 0)
C[v] = cu[v];
for(auto u : g[v]){
if(u == p) continue;
if(dp[u] == 0)
C[v] += cd[u];
}
}
}
for(auto u : g[v])
if(u != p)
dfs2(u, v);
}
void solve(){
cin >> n >> d;
for(int i = 2; i <= n; i++){
int u, v;
cin >> u >> v;
g[v].pb(u);
g[u].pb(v);
}
dfs(1, 0);
ndp[1] = 1;
cu[1] = 0;
dfs1(1, 0);
dfs2(1, 0);
int L = 0;
for(int i = 1; i <= n; i++)
L += (st[i] == 0);
int E = 0;
for(int i = 1; i <= n; i++){
if(st[i])
E += C[i];
else
E -= C[i];
}
int n2 = mult(n, n);
int lst = mult(L, mult(sum(binpow(n2, d), -binpow(E, d)), binpow(sum(n2, -E), mod - 2)));
int ret;
if(st[1]){
ret = sum(binpow(n2, d), -mult(C[1], lst));
}else{
ret = mult(C[1], lst);
}
cout << ret << '\n';
}
signed main() {
mispertion;
int t = 1;
//cin >> t;
while(t--){
solve();
}
return 0;
}
