#include <bits/stdc++.h>
using namespace std;
#define rep(i, n) for(int i = 0; i < n; i++)
#define rep2(i, x, n) for(int i = x; i <= n; i++)
#define rep3(i, x, n) for(int i = x; i >= n; i--)
#define elif else if
#define sp(x) fixed << setprecision(x)
#define pb push_back
#define eb emplace_back
#define all(x) x.begin(), x.end()
#define sz(x) (int)x.size()
using ll = long long;
using pii = pair<int, int>;
using pil = pair<int, ll>;
using pli = pair<ll, int>;
using pll = pair<ll, ll>;
const ll MOD = 1000000007;
//const ll MOD = 998244353;
const int inf = (1<<30)-1;
const ll INF = (1LL<<60)-1;
const double pi = acos(-1.0);
const double EPS = 1e-10;
template<typename T> bool chmax(T &x, const T &y) {return (x < y)? (x = y, true) : false;};
template<typename T> bool chmin(T &x, const T &y) {return (x > y)? (x = y, true) : false;};

template<ll mod>
struct Mod_Int{
    ll x;

    Mod_Int() = default;

    Mod_Int(ll y) : x(y >= 0 ? y % mod : (mod - (-y) % mod) % mod) {}

    Mod_Int &operator += (const Mod_Int &p){
        x = (x + p.x) % mod;
        return *this;
    }

    Mod_Int &operator -= (const Mod_Int &p){
        x = (x + mod - p.x) % mod;
        return *this;
    }

    Mod_Int &operator *= (const Mod_Int &p){
        x = (x * p.x) % mod;
        return *this;
    }

    Mod_Int &operator /= (const Mod_Int &p){
        *this *= p.inverse();
        return *this;
    }

    Mod_Int &operator ++ () {return *this += Mod_Int(1);}

    Mod_Int operator ++ (int){
        Mod_Int tmp = *this;
        ++*this;
        return tmp;
    }

    Mod_Int &operator -- () {return *this -= Mod_Int(1);}

    Mod_Int operator -- (int){
        Mod_Int tmp = *this;
        --*this;
        return tmp;
    }

    Mod_Int operator - () const {return Mod_Int(-x);}

    Mod_Int operator + (const Mod_Int &p) const {return Mod_Int(*this) += p;}

    Mod_Int operator - (const Mod_Int &p) const {return Mod_Int(*this) -= p;}

    Mod_Int operator * (const Mod_Int &p) const {return Mod_Int(*this) *= p;}

    Mod_Int operator / (const Mod_Int &p) const {return Mod_Int(*this) /= p;}

    bool operator == (const Mod_Int &p) const {return x == p.x;}

    bool operator != (const Mod_Int &p) const {return x != p.x;}

    Mod_Int inverse() const {
        assert(*this != Mod_Int(0));
        return pow(mod-2);
    }

    Mod_Int pow(ll k) const{
        Mod_Int now = *this, ret = 1;
        while(k){
            if(k&1) ret *= now;
            now *= now, k >>= 1;
        }
        return ret;
    }

    friend ostream &operator << (ostream &os, const Mod_Int &p){
        return os << p.x;
    }

    friend istream &operator >> (istream &is, Mod_Int &p){
        ll a;
        is >> a;
        p = Mod_Int<mod>(a);
        return is;
    }
};

using mint = Mod_Int<MOD>;

struct Graph{
    vector<vector<int>> es;
    const int n;
    vector<vector<mint>> memo;

    Graph(int n) : n(n){
        es.resize(n);
        memo.assign(n, vector<mint>(n+1, -1));
    }

    void add_edge(int from, int to, bool directed = false){
        es[from].pb(to);
        if(!directed) es[to].pb(from);
    }

    mint dfs(int now, int pre, int c, int C){
        if(memo[now][c] != -1) return memo[now][c];
        mint ret = 1;
        for(auto &e: es[now]){
            if(e == pre) continue;
            mint tmp = 0;
            if(c == n){
                tmp += dfs(e, now, n, C)*2;
            }
            else{
                if(c >= 1) tmp += dfs(e, now, c-1, C);
                int nc = min(c+1, C-1-(c+1));
                if(nc >= 0) tmp += dfs(e, now, nc, C);
            }
            ret *= tmp;
        }
        return memo[now][c] = ret;
    }

    mint solve(int c){
        rep(i, n) rep(j, n+1) memo[i][j] = -1;
        mint ret = 0;
        if(c <= 2*n){
            rep(i, c){
                ret += dfs(0, -1, min(i, c-1-i), c);
            }
            return ret;
        }
        rep(i, n){
            ret += dfs(0, -1, i, c)*2;
        }
        ret += dfs(0, -1, n, c)*(c-2*n);
        return ret;
    }
};

int main(){
    int N, C;
    cin >> N >> C;
    Graph G(N);
    rep(i, N-1){
        int u, v; cin >> u >> v; u--, v--;
        G.add_edge(u, v);
    }
    mint ans = 0;
    rep(i, 3){
        ans += G.solve((C+i)/3);
    }
    cout << ans << endl;
}