#include <bits/stdc++.h>
#define int long long
using namespace std;
const int N=100010, Mod=1e9+7;
int n, d, dp[N], ep[N], cx[N], cy[N], cnt[N], w[N], c[N], c1, c2, ans[N], rans;
vector<int> adj[N];
void dfs1(int curr, int prev) {
for(int next:adj[curr]) if(next!=prev) {
dfs1(next, curr);
cnt[curr]+=1-dp[next];
if(!dp[next]) dp[curr]=true;
}
}
void dfs2(int curr, int prev) {
for(int next:adj[curr]) if(next!=prev) {
if(cnt[curr]-(1-dp[next])==0 && !ep[curr]) ep[next]=true;
dfs2(next, curr);
}
w[curr]=dp[curr]|ep[curr];
}
void dfs3(int curr, int prev) {
cx[curr]=1-dp[curr];
for(int next:adj[curr]) if(next!=prev) dfs3(next, curr);
if(dp[curr]) {
vector<int> v;
for(int next:adj[curr]) if(next!=prev && !dp[next]) v.push_back(next);
if(v.size()==1) cx[curr]+=cx[v[0]];
}
else {
for(int next:adj[curr]) if(next!=prev) cx[curr]+=cx[next];
}
}
void dfs4(int curr, int prev) {
cy[curr]+=1-dp[curr];
for(int next:adj[curr]) if(next!=prev) {
if(!w[next] || (cnt[next]+ep[next]==1 && !dp[next])) {
cy[next]=cy[curr]+cx[curr]-(1-dp[curr]);
if(!dp[curr] || dp[curr] && cnt[curr]==1 && !dp[next]) cy[next]-=cx[next];
}
dfs4(next, curr);
}
if(!w[curr]) c[curr]=cx[curr]+cy[curr]-(1-dp[curr]);
else if(cnt[curr]+ep[curr]>=2) c[curr]=0;
else if(dp[curr]) c[curr]=cx[curr];
else c[curr]=cy[curr];
}
signed main() {
ios_base::sync_with_stdio(0); cin.tie(0);
cin>>n>>d;
for(int i=1; i<n; i++) {
int u, v; cin>>u>>v;
adj[u].push_back(v), adj[v].push_back(u);
}
dfs1(1, 0), dfs2(1, 0), dfs3(1, 0), dfs4(1, 0);
for(int i=1; i<=n; i++) ans[0]+=w[i];
for(int i=1; i<=n; i++) c1+=w[i], c2+=c[i]*(1-2*w[i]);
int val=1;
for(int i=1; i<d; i++, val=(val*n)%Mod, val=(val*n)%Mod) {
ans[i]=(c1*val)%Mod, ans[i]=(ans[i]*n)%Mod, ans[i]=(ans[i]*n)%Mod;
ans[i]=(ans[i]+((val*n)%Mod)*c2)%Mod, ans[i]=(ans[i]-ans[i-1]*c2)%Mod, ans[i]=(ans[i]+Mod)%Mod;
}
val=(val*n)%Mod, rans=(c[1]*(val-ans[d-1]+Mod))%Mod;
if(w[1]) val=(val*n)%Mod, rans=(val-rans+Mod)%Mod;
cout<<rans;
return 0;
}
