#include <iostream>
#include <algorithm>
#include <iomanip>
#include <vector>
#include <queue>
#include <set>
#include <map>
#include <tuple>
#include <cmath>
#include <numeric>
#include <functional>
#include <cassert>

#define debug_value(x) cerr << "line" << __LINE__ << ":<" << __func__ << ">:" << #x << "=" << x << endl;
#define debug(x) cerr << "line" << __LINE__ << ":<" << __func__ << ">:" << x << endl;

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; }

using namespace std;
typedef long long ll;

const ll MOD = 1000000007;

class ModInt{
    public:
    ll v;
    ModInt(ll _v = 0){
        if(_v >= MOD) _v %= MOD;
        v = _v;
    }
    ModInt operator+(ll n){
        return ModInt((v+n)%MOD);
    }
    ModInt operator-(ll n){
        return ModInt((v-n+MOD)%MOD);
    }
    ModInt operator*(ll n){
        if(n >= MOD) n %= MOD;
        return ModInt((v*n)%MOD);
    }
    ModInt operator/(ll n){
        return ModInt((ModInt(n).inv()*v).v%MOD);
    }

    void operator+=(ll n){
        v = (v+n)%MOD;
    }
    void operator-=(ll n){
        v = (v-n+MOD)%MOD;
    }
    void operator*=(ll n){
        v = (v*n+MOD)%MOD;
    }
    
    
    ModInt operator+(ModInt n){
        return ModInt((v+n.v)%MOD);
    }
    ModInt operator-(ModInt n){
        return ModInt((v-n.v+MOD)%MOD);
    }
    ModInt operator*(ModInt n){
        return ModInt((v*n.v)%MOD);
    }
    ModInt operator/(ModInt n){
        return ModInt((n.inv()*v).v%MOD);
    }

    void operator+=(ModInt n){
        v = (v+n.v)%MOD;
    }
    void operator-=(ModInt n){
        v = (v-n.v+MOD)%MOD;
    }
    void operator*=(ModInt n){
        v = (v*n.v)%MOD;
    }

    void operator=(ModInt n){
        v = n.v;
    }
    bool operator==(ModInt n){
        return v == n.v;
    }
    bool operator!=(ModInt n){
        return v != n.v;
    }
    void operator=(ll n){
        v = n%MOD;
    }

    ModInt inv(){
        if(v == 1) return ModInt(1);
        else return ModInt(MOD-ModInt(MOD%v).inv().v*(MOD/v)%MOD);
    }
};

ostream& operator<<(ostream& os, const ModInt& m){
    os << m.v;
    return os;
}

istream & operator >> (istream &in,  ModInt &m){
    in >> m.v;
    return in;
}

ModInt pow(ModInt a, ll n) {
    assert(n >= 0);
	ModInt ans = 1;
	ModInt tmp = a;
	for (int i = 0; i <= 60; i++) {
		ll m = (ll)1 << i;
		if (m & n) {
		ans *= tmp;
		}
		tmp *= tmp;
	}
	return ans;
}

using mint = ModInt;

bool used[1005];

void dfs(int v, vector<vector<int>> &g_undirected, vector<int> &par){
    used[v] = true;
    for(int to : g_undirected[v]){
        if(used[to]) continue;
        par[to] = v;
        dfs(to, g_undirected, par);
    }
}

bool ok[1005][12005];
mint memo[1005][12005];

mint solve(int v, vector<vector<int>> &g, int p, int c){
    if(ok[v][p]){
        return memo[v][p]; 
    }
    mint ans = 1;
    for(int to : g[v]){
        mint tmp = 0;
        if(p-3 >= 1) tmp += solve(to, g, p-3, c);
        if(p+3 <= c) tmp += solve(to, g, p+3, c);
        ans *= tmp;
    }
    ok[v][p] = true;
    memo[v][p] = ans;
    return ans;
}

int main(){
    ios::sync_with_stdio(false);
    cin.tie(0);
    cout << setprecision(10) << fixed;
    int n, c; cin >> n >> c;
    vector<vector<int>> g_undirected(n);
    vector<vector<int>> g_directed(n);
    for(int i = 0; i < n-1; i++){
        int a, b; cin >> a >> b; a--; b--;
        g_undirected[a].push_back(b);
        g_undirected[b].push_back(a);
    }
    vector<int> par(n);
    dfs(0, g_undirected, par);
    mint ans = 0;
    if(c >= 3*n){
        if(c-3*n > 3*n+1) ans += pow(mint(2), n-1)*(c-3*n - (3*n+1) -1);
    }
    for(int i = 1; i < n; i++){
        g_directed[par[i]].push_back(i);
    }
    for(int i = 1; i <= 3*n && i <= c; i++) {
        ans += solve(0, g_directed, i, c);
    }
    if(c-3*n > 0){
        for(int i = c-3*n; i <= c; i++){
            if(i <= 3*n) continue;
            ans += solve(0, g_directed, c-i+1, c);
        }
    }
    cout << ans << endl;
}