#include <bits/stdc++.h>
#define rep(i,n) for (int i = 0; i < (int)(n); i ++)
#define irep(i,n) for (int i = (int)(n) - 1;i >= 0;--i)
using namespace std;
using ll = long long;
using PL = pair<ll,ll>;
using P = pair<int,int>;
constexpr int INF = 1000000000;
constexpr long long HINF = 1000000000000000;
constexpr long long MOD = 1000000007;// = 998244353;
constexpr double EPS = 1e-4;
constexpr double PI = 3.14159265358979;

template<int Modulus> 
struct ModInt {
    long long x;
    ModInt(long long x = 0) :x((x%Modulus + Modulus)%Modulus) {}
    constexpr ModInt &operator+=(const ModInt a) {if ((x += a.x) >= Modulus) x -= Modulus; return *this;}
    constexpr ModInt &operator-=(const ModInt a) {if ((x += Modulus - a.x) >= Modulus) x -= Modulus; return *this;}
    constexpr ModInt &operator*=(const ModInt a) {(x *= a.x) %= Modulus; return *this;}
    constexpr ModInt &operator/=(const ModInt a) {return *this *= a.inverse();}

    constexpr ModInt operator+(const ModInt a) const {return ModInt(*this) += a.x;}
    constexpr ModInt operator-(const ModInt a) const {return ModInt(*this) -= a.x;}
    constexpr ModInt operator*(const ModInt a) const {return ModInt(*this) *= a.x;}
    constexpr ModInt operator/(const ModInt a) const {return ModInt(*this) /= a.x;}
    
    friend constexpr ostream& operator<<(ostream& os,const ModInt<Modulus>& a) {return os << a.x;}
    friend constexpr istream& operator>>(istream& is,ModInt<Modulus>& a) {return is >> a.x;}
    
    ModInt inverse() const {// x ^ (-1) 
        long long a = x,b = Modulus,p = 1,q = 0;
        while (b) {long long d = a/b; a -= d*b; swap(a,b); p -= d*q; swap(p,q);}
        return ModInt(p);
    }
    ModInt pow(long long N) {// x ^ N 
        ModInt a = 1;
        while (N) {
            if (N&1) a *= *this;
            *this *= *this;
            N >>= 1;
        }
        return a;
    }
};

using mint = ModInt<1000000007>;

int N,M;
vector<vector<int>> G(N);
vector<vector<mint>> dp;
vector<vector<int>> visited;

mint dfs(int v,int c,int p = -1) {
    if (visited[v][c] >= 0) return dp[v][c];
    visited[v][c] = 1;
    mint ans = 1;
    for (int e:G[v]) {
        if (e == p) continue;
        mint ret = 0;
        if (c - 3 > 0) ret += dfs(e,c-3,v);
        if (c + 3 <= M) ret += dfs(e,c+3,v);
        ans *= ret;
    }
    return dp[v][c] = ans;
}

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

    ll C;
    cin >> N >> C;
    M = min((ll)6*N + 10,C);
    G.resize(N);
    dp.assign(N,vector<mint>(M + 1,0));
    visited.assign(N,vector<int>(M + 1,-1));
    rep(i,N - 1) {
        int a,b; cin >> a >> b;
        --a; --b;
        G[a].push_back(b);
        G[b].push_back(a);
    }

    int K = min((ll)3*N + 3,C);
    rep(i,K) dfs(0,i + 1);
    mint ans = 0;
    if (C <= K) {
        rep(i,C) ans += dp[0][i + 1];
        cout << ans << '\n';
    } else if (C < 2*K){
        rep(i,K) ans += dp[0][i + 1];
        rep(i,C - K) ans += dp[0][i + 1];
        cout << ans << '\n';
    } else {
        rep(i,K) ans += mint(2)*dp[0][i + 1];
        ans += mint(C - 2*K) * dp[0][K];
        cout << ans << '\n';
    }
    return 0;
}