//#define _GLIBCXX_DEBUG
#include <bits/stdc++.h>
#define rep(i, n) for(int i=0; i<n; ++i)
#define all(v) v.begin(), v.end()
#define rall(v) v.rbegin(), v.rend()
using namespace std;
using ll = int64_t;
using ld = long double;
using P = pair<int, int>;
using vs = vector<string>;
using vi = vector<int>;
using vvi = vector<vi>;
template<class T> using PQ = priority_queue<T>;
template<class T> using PQG = priority_queue<T, vector<T>, greater<T> >;
const int INF = 0xccccccc;
const ll LINF = 0xcccccccccccccccLL;
template<typename T1, typename T2>
inline bool chmax(T1 &a, T2 b) {return a < b && (a = b, true);}
template<typename T1, typename T2>
inline bool chmin(T1 &a, T2 b) {return a > b && (a = b, true);}
template<typename T1, typename T2>
istream &operator>>(istream &is, pair<T1, T2> &p) { return is >> p.first >> p.second;}
template<typename T1, typename T2>
ostream &operator<<(ostream &os, const pair<T1, T2> &p) { return os << p.first << ' ' << p.second;}

const unsigned int mod = 1000000007;
//const unsigned int mod = 998244353;

struct mint {
	unsigned int x;
	mint():x(0) {}
	mint(int64_t x_) {
		int64_t v = int64_t(x_ % mod);
		if(v < 0) v += mod;
		x = (unsigned int)v;
	}
	static mint row(int v) {
		mint v_;
		v_.x = v;
		return v_;
	}
	mint operator-() const { return mint(-x);}
	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) {
		uint64_t z = x;
		z *= a.x;
		x = (unsigned int)(z % mod);
		return *this;
	}
	mint operator+(const mint a) const { return mint(*this) += a;}
	mint operator-(const mint a) const { return mint(*this) -= a;}
	mint operator*(const mint a) const { return mint(*this) *= a;}
	friend bool operator==(const mint &a, const mint &b) {return a.x == b.x;}
	friend bool operator!=(const mint &a, const mint &b) {return a.x != b.x;}
	mint &operator++() {
		x++;
		if(x == mod) x = 0;
		return *this;
	}
	mint &operator--() {
		if(x == 0) x = mod;
		x--;
		return *this;
	}
	mint operator++(int) {
		mint result = *this;
		++*this;
		return result;
	}
	mint operator--(int) {
		mint result = *this;
		--*this;
		return result;
	}
	mint pow(int64_t t) const {
		mint x_ = *this, r = 1;
		while(t) {
			if(t&1) r *= x_;
			x_ *= x_;
			t >>= 1;
		}
		return r;
	}
	//for prime mod
	mint inv() const { return pow(mod-2);}
	mint& operator/=(const mint a) { return *this *= a.inv();}
	mint operator/(const mint a) {return mint(*this) /= a;}
};

istream& operator>>(istream& is, mint& a) { return is >> a.x;}
ostream& operator<<(ostream& os, const mint& a) { return os << a.x;}

#define N 1010

//head

int n, c;
vi G[N];
mint dp[N][N];
mint ans;

int main() {
	ios::sync_with_stdio(false);
	cin.tie(0);
	cin >> n >> c;
	rep(i, n-1) {
		int a, b;
		cin >> a >> b;
		a--; b--;
		G[a].emplace_back(b);
		G[b].emplace_back(a);
	}
	memset(dp, -1, sizeof(dp));
	if(c > N*2*3) {
		auto dfs = [&](int i, int j, int x, auto self)->mint {
			if(dp[i][x].x != 4294967295U) return dp[i][x];
			mint &res = dp[i][x] = 1;
			if(x == N) {
				for(int ne:G[i]) if(ne != j) {
					res *= self(ne, i, N, self)*2;
				}
			}
			else {
				if(x == 1) {
					for(int ne:G[i]) if(ne != j) res *= self(ne, i, 2, self);
				}
				else {
					for(int ne:G[i]) if(ne != j) res *= self(ne, i, x-1, self) + self(ne, i, x+1, self);
				}
			}
			return res;
		};
		for(int i = 1; i < N; i++) {
			ans += dfs(0, -1, i, dfs) * 6;
		}
		ans += dfs(0, -1, N, dfs) * (c-N*6+6);
	}
	else {
		auto dfs = [&](int i, int j, int x, auto self)->mint {
			if(dp[i][x].x != 4294967295U) return dp[i][x];
			mint &res = dp[i][x] = 1;
			for(int ne:G[i]) if(ne != j) {
				mint now = 0;
				if(x-3 > 0) now += self(ne, i, x-3, self);
				if(x+3 < c+1) now += self(ne, i, x+3, self);
				res *= now;
			}
			return res;
		};
		for(int i = 1; i <= c; i++) {
			ans += dfs(0, -1, i, dfs);
		}
	}
	cout << ans << endl;
}