#include "bits/stdc++.h"
using namespace std;
#define ar array
typedef int64_t ll;
const int N = 1e5 + 5;
const int mod = 1e9 + 7;
vector<int> edges[N];
int dp[N], up[N], cnt[N], n;
int sub[N], rev[N], tot[N], tmp[N];
void dfs(int u, int p = -1){
for(auto x : edges[u]){
if(x == p) continue;
dfs(x, u);
if(dp[x] == 0){
dp[u] = 1,
tmp[u] += sub[x],
cnt[u]++;
}
tot[u] += sub[x];
}
if(cnt[u] > 1) sub[u] = 0;
if(cnt[u] == 1) sub[u] = tmp[u];
if(cnt[u] == 0) sub[u] = tot[u] + 1;
}
void re(int u, int p = -1){
tot[u] += rev[u];
if(up[u] == 0){
tmp[u] += rev[u];
dp[u] = 1, cnt[u]++;
}
if(cnt[u] > 1) sub[u] = 0;
if(cnt[u] == 1) sub[u] = tmp[u];
if(cnt[u] == 0) sub[u] = tot[u] + 1;
for(auto x : edges[u]){
if(x == p) continue;
tot[u] -= sub[x];
if(!dp[x]){
cnt[u]--, tmp[u] -= sub[x];
}
if(cnt[u]) up[x] = 1;
if(cnt[u] > 1) rev[x] = 0;
if(cnt[u] == 1) rev[x] = tmp[u];
if(cnt[u] == 0) rev[x] = tot[u] + 1;
tot[u] += sub[x];
if(!dp[x]){
cnt[u]++, tmp[u] += sub[x];
}
re(x, u);
}
}
signed main(){
ios::sync_with_stdio(0); cin.tie(0);
int d; cin>>n>>d;
for(int i=1;i<n;i++){
int a, b; cin>>a>>b;
edges[a].push_back(b);
edges[b].push_back(a);
}
dfs(1);
up[1] = 1;
re(1);
int E = 0, L = 0;
for(int i=1;i<=n;i++){
if(dp[i] == 1) E = (E + sub[i]) % mod;
else E = (E - sub[i] + mod) % mod, L++;
}
//~ for(int i=1;i<=n;i++){
//~ cout<<sub[i]<<" ";
//~ } cout<<"\n";
vector<int> e(d + 1), f(d + 1);
e[0] = f[0] = 1;
for(int i=1;i<=d;i++){
e[i] = e[i-1] * 1ll * E % mod;
f[i] = f[i-1] * 1ll * n % mod * n % mod;
}
d--;
int res = 0;
for(int i=0;i<=d;i++){
res = (res + f[d-i] * 1ll * e[i]) % mod;
}
res = res * 1ll * L % mod;
if(dp[1]){
res = res * 1ll * sub[1] % mod;
} else {
res = (f[d + 1] - res * 1ll * sub[1] % mod + mod) % mod;
}
res = (f[d + 1] - res + mod) % mod;
cout<<res<<"\n";
}
/*
5 1
1 2
2 3
2 4
1 5
*/
