#include <iostream>
#include <string>
#include <sstream>
#include <stack>
#include <algorithm>
#include <cmath>
#include <queue>
#include <bitset>
#include <iomanip>
#include <limits>
#include <chrono>
#include <random>
#include <array>
#include <unordered_map>
#include <functional>
#include <complex>
#include <numeric>
#include <cctype>
#include <map>
#include <set>
#include <cstdlib>
#include <bitset>
#include <tuple>
#include <assert.h>
#include <deque>
#include <utility>
#include <fstream>

using namespace std;
typedef long long ll;

template<class T> inline bool chmax(T& a, T b) { if (a < b) { a = b; return 1; } return 0; }
template<class T> inline bool chmin(T& a, T b) { if (a > b) { a = b; return 1; } return 0; }
template<typename T> T gcd(T a, T b) { a = abs(a), b = abs(b); while (b > 0) { tie(a, b) = make_pair(b, a % b); } return a; }
//mt19937 rnd(chrono::steady_clock::now().time_since_epoch().count());

constexpr long long INF = 1LL << 60;
constexpr int inf = 1000000007;
constexpr long long mod = 1000000007LL;
constexpr int MAX = 5000000;
struct mint {
	long long x;
	mint(long long x = 0) :x((x% mod + mod) % mod) {}
	mint& operator+=(const mint a) {
		if ((x += a.x) >= mod) x -= mod;
		return *this;
	}
	mint& operator-=(const mint a) {
		if ((x += mod - a.x) >= mod) x -= mod;
		return *this;
	}
	mint& operator*=(const mint a) {
		(x *= a.x) %= mod;
		return *this;
	}
	mint operator+(const mint a) const {
		mint res(*this);
		return res += a;
	}
	mint operator-(const mint a) const {
		mint res(*this);
		return res -= a;
	}
	mint operator*(const mint a) const {
		mint res(*this);
		return res *= a;
	}
	mint pow(ll t) const {
		if (!t) return 1;
		mint a = pow(t >> 1);
		a *= a;
		if (t & 1) a *= *this;
		return a;
	}

	// for prime mod
	mint inv() const {
		return pow(mod - 2);
	}
	mint& operator/=(const mint a) {
		return (*this) *= a.inv();
	}
	mint operator/(const mint a) const {
		mint res(*this);
		return res /= a;
	}
};

int main()
{
	/*
	cin.tie(nullptr);
	ios::sync_with_stdio(false);
	*/

	int n, C; cin >> n >> C;
	vector<vector<int>> g(n);
	for (int i = 0; i < n - 1; i++) {
		int u, v; cin >> u >> v;
		u--; v--;
		g[u].emplace_back(v);
		g[v].emplace_back(u);
	}
	int cnt = 0;
	if (n * 10 < C) {
		cnt = C - n * 10;
		C = n * 10;
	}
	vector<vector<mint>> dp(n, vector<mint>(C));
	auto dfs = [&](auto&& f, int cur, int pre)->void {
		for (int i = 0; i < C; i++) {
			dp[cur][i] = 1;
		}
		for (auto nxt : g[cur]) {
			if (nxt == pre) continue;
			f(f, nxt, cur);
			for (int i = 0; i < C; i++) {
				mint cnt = 0;
				if (i - 3 >= 0) {
					cnt += dp[nxt][i - 3];
				}
				if (i + 3 < C) {
					cnt += dp[nxt][i + 3];
				}
				dp[cur][i] *= cnt;
			}
		}
	};
	dfs(dfs, 0, -1);
	mint res = 0;
	for (int i = 0; i < C; i++) res += dp[0][i];
	res += mint(2).pow(n - 1) * cnt;
	cout << res.x << "\n";
	return 0;
}