#include <iostream>
#include <vector>
#include <algorithm>
#include <cmath>
#include <string>
#include <queue>
#include <stack>
#include <set>
#include <map>
#include <iomanip>
#include <utility>
#include <tuple>
#include <functional>
#include <bitset>
#include <cassert>
#include <complex>
#include <stdio.h>
#include <time.h>
#include <numeric>
#include <random>
#include <unordered_map>
#include <unordered_set>
#define all(a) a.begin(),a.end()
#define rep(i, n) for (ll i = 0; i < (n); i++)
#define pb push_back
#define debug(x) cerr << __LINE__ << ' ' << #x << ':' << x << '\n'
#pragma GCC optimize("O3")
#pragma GCC optimize("unroll-loops")
using namespace std;
typedef long long ll;
typedef unsigned int uint;
typedef unsigned long long ull;
typedef long double ld;
typedef pair<ll, ll> P;
typedef complex<ld> com;
template<class T> using pque = priority_queue<T, vector<T>, greater<T>>;
constexpr int inf = 1000000010;
constexpr ll INF = 1000000000000000010;
constexpr int mod1e9 = 1000000007;
constexpr int mod998 = 998244353;
constexpr ld eps = 1e-12;
constexpr ld pi = 3.141592653589793238;
constexpr ll ten(int n) { return n ? 10 * ten(n - 1) : 1; };
int dx[] = { 1,0,-1,0,1,1,-1,-1 }; int dy[] = { 0,1,0,-1,1,-1,1,-1 };
void fail() { cout << "-1\n"; exit(0); }
template<class T, class U> inline bool chmax(T &a, const U &b) { if (a < b) { a = b; return true; } return false; }
template<class T, class U> inline bool chmin(T &a, const U &b) { if (a > b) { a = b; return true; } return false; }
template<class T> istream &operator >> (istream &s, vector<T> &v) { for (auto &e : v) s >> e; return s; }
template<class T> ostream &operator << (ostream &s, const vector<T> &v) { for (auto &e : v) s << e << ' '; return s; }

struct fastio {
	fastio() {
		cin.tie(0); cout.tie(0);
		ios::sync_with_stdio(false);
		cout << fixed << setprecision(20);
		cerr << fixed << setprecision(20);
	}
}fastio_;

constexpr ll mod = mod1e9;
ll dp[3010][6030];
vector<vector<int>> graph(1010, vector<int>());

int idx(int n, int c) {
	if (c <= 6020) return n;
	if (n < 3010) return n;
	if (3010 <= n && n <= c - 3011) return 6020;
	if (c - 3010 <= n && n <= c - 1) return 6020 - c + n;
}

int c;

void dfs(int n, int p) {
	for (int i : graph[n]) {
		if (i == p) continue;
		dfs(i, n);
	}
	rep(i, min(6021, c)) dp[n][i] = 1;
	for (int i : graph[n]) {
		if (i == p) continue;
		rep(j, c) {
			if (c > 6020 && j == 3010) {
				dp[n][idx(j, c)] *= (2 * dp[i][idx(j, c)]);
				dp[n][idx(j, c)] %= mod;
				j = c - 3011;
			}
			else if (j <= 2) {
				dp[n][idx(j, c)] *= dp[i][idx(j + 3, c)];
				dp[n][idx(j, c)] %= mod;
			}
			else if (j >= c - 3) {
				dp[n][idx(j, c)] *= dp[i][idx(j - 3, c)];
				dp[n][idx(j, c)] %= mod;
			}
			else {
				dp[n][idx(j, c)] *= (dp[i][idx(j - 3, c)] + dp[i][idx(j + 3, c)]);
				dp[n][idx(j, c)] %= mod;
			}
		}
	}
}

int main() {
	int n;
	cin >> n >> c;
	rep(i, n - 1) {
		int u, v;
		cin >> u >> v;
		u--; v--;
		graph[u].pb(v);
		graph[v].pb(u);
	}
	dfs(0, -1);
	ll ans = 0;
	rep(i, 6020) ans += dp[0][i];
	if (c > 6020) ans += dp[0][6020] * (c - 6020);
	cout << ans % mod << '\n';
}