#include <bits/stdc++.h>
#define int long long
using namespace std;
#define fastIO ios_base::sync_with_stdio(0);cin.tie(0);cout.tie(0);
#define pb push_back
#define pii pair<int, int>
#define S second
#define F first
#define sep ' '
#define vi vector<int>
#define mid (l+r)/2
#define pbb pair<bool, bool>
#define trint array<int, 3>
const int N = 1e3+10, M = 1e9+7;
int n, d;
vi adj[N];
int exi, stat;
int cnt[2][2];
bool dfs(int v, int p = -1) {
if (v == exi && stat == false) return true;
for(int neigh: adj[v]) {
if (neigh == p) continue;
if (dfs(neigh, v) == false) return true;
}
return false;
}
void solve() {
cin >> n >> d;
assert(n <= 1e3 && d <= 1e5);
for(int i = 0 ; i < n-1 ; i ++) {
int u, v; cin >> u >> v;u --, v --; adj[u].pb(v), adj[v].pb(u);
}
for(int root = 0 ; root < n ; root ++) {
for(exi = 0; exi < n ; exi ++) {
for(stat = 0; stat <= 1; stat++) {
bool res = dfs(root);
cnt[res][stat]++;
}
}
}
// cout << "#\n";
exi = -1;
vi base(2);
for(int root = 0 ; root < n ; root ++) {
base[dfs(root)]++;
}
for(int i = 1 ; i <= d-1 ; i ++) {
base = {
(1ll * base[0] * cnt[0][0] % M + 1ll * base[1] * cnt[0][1] % M) % M,
(1ll * base[0] * cnt[1][0] % M + 1ll * base[1] * cnt[1][1] % M) % M,
};
}
int ans = 0;
for(exi = 0; exi < n ; exi ++) {
for(stat = 0; stat <= 1; stat++) {
if (dfs(0)) {
ans += base[stat];
ans %= M;
}
}
}
cout << ans << endl;
}
int32_t main() {
fastIO;
solve();
return 0;
}
