#line 1 "code.cpp"
#define MOD 1000000007LL
//#define MOD 998244353LL
//#define MOD 18446744069414584321ULL
#include<bits/stdc++.h>
#line 1 "cpplib/util/template.hpp"
#pragma GCC optimize("Ofast")
#pragma GCC optimize("unroll-loops")
#pragma GCC target("avx")
#line 5 "cpplib/util/template.hpp"
using namespace std;
__attribute__((constructor))void init(){cin.tie(0);ios::sync_with_stdio(false);cout<<fixed<<setprecision(15);}
typedef long long lint;
#define INF (1LL<<60)
#define IINF (1<<30)
#define EPS (1e-10)
#define endl ('\n')
typedef vector<lint> vec;
typedef vector<vector<lint>> mat;
typedef vector<vector<vector<lint>>> mat3;
typedef vector<string> svec;
typedef vector<vector<string>> smat;
template<typename T>inline void numout(T t){bool f=0;for(auto i:t){cout<<(f?" ":"")<<i<INF/2?i:"INF";f=1;}cout<<endl;}
template<typename T>inline void numout2(T t){for(auto i:t)numout(i);}
template<typename T>inline void output(T t){bool f=0;for(auto i:t){cout<<(f?" ":"")<<i;f=1;}cout<<endl;}
template<typename T>inline void output2(T t){for(auto i:t)output(i);}
template<typename T>inline void _output(T t){bool f=0;for(lint i=0;i<t.size();i++){cout<<f?"":" "<<t[i];f=1;}cout<<endl;}
template<typename T>inline void _output2(T t){for(lint i=0;i<t.size();i++)output(t[i]);}
#define rep(i,...) for(auto i:range(__VA_ARGS__)) 
#define rrep(i,...) for(auto i:reversed(range(__VA_ARGS__)))
#define repi(i,a,b) for(lint i=lint(a);i<(lint)(b);++i)
#define rrepi(i,a,b) for(lint i=lint(b)-1;i>=lint(a);--i)
#define irep(i) for(lint i=0;;++i)
inline vector<long long> range(long long n){vector<long long>v(n);iota(v.begin(),v.end(),0LL);return v;}
inline vector<long long> range(long long a,long long b){vector<long long>v(b-a);iota(v.begin(),v.end(),a);return v;}
inline vector<long long> range(long long a,long long b,long long c){if((b-a+c-1)/c<=0)return vector<long long>();vector<long long>v((b-a+c-1)/c);for(int i=0;i<(int)v.size();++i)v[i]=i?v[i-1]+c:a;return v;}
template<typename T>inline T reversed(T v){reverse(v.begin(),v.end());return v;}
#define all(n) begin(n),end(n)
#define dist(a,b,c,d) sqrt(pow(a-c,2)+pow(b-d,2))
template<typename T,typename E>bool chmin(T& s,const E& t){bool res=s>t;s=min<T>(s,t);return res;}
template<typename T,typename E>bool chmax(T& s,const E& t){bool res=s<t;s=max<T>(s,t);return res;}
const vector<lint> dx={1,0,-1,0,1,1,-1,-1};
const vector<lint> dy={0,1,0,-1,1,-1,1,-1};
#define SUM(v) accumulate(all(v),0LL)
template<typename T,typename ...Args>auto make_vector(T x,int arg,Args ...args){if constexpr(sizeof...(args)==0)return vector<T>(arg,x);else return vector(arg,make_vector<T>(x,args...));}
#line 1 "cpplib/math/mod_int.hpp"
class mint {
  using u64 = std::uint_fast64_t;
    public:
    u64 a;
    constexpr mint(const long long x = 0)noexcept:a(x>=0?x%get_mod():get_mod()-(-x)%get_mod()){}
    constexpr u64 &value()noexcept{return a;}
    constexpr const u64 &value() const noexcept {return a;}
    constexpr mint operator+(const mint rhs)const noexcept{return mint(*this) += rhs;}
    constexpr mint operator-(const mint rhs)const noexcept{return mint(*this)-=rhs;}
    constexpr mint operator*(const mint rhs) const noexcept {return mint(*this) *= rhs;}
    constexpr mint operator/(const mint rhs) const noexcept {return mint(*this) /= rhs;}
    constexpr mint &operator+=(const mint rhs) noexcept {
        a += rhs.a;
        if (a >= get_mod())a -= get_mod();
        return *this;
    }
    constexpr mint &operator-=(const mint rhs) noexcept {
        if (a<rhs.a)a += get_mod();
        a -= rhs.a;
        return *this;
    }
    constexpr mint &operator*=(const mint rhs) noexcept {
        a = a * rhs.a % get_mod();
        return *this;
    }
    constexpr mint operator++(int) noexcept {
        a += 1;
        if (a >= get_mod())a -= get_mod();
        return *this;
    }
    constexpr mint operator--(int) noexcept {
        if (a<1)a += get_mod();
        a -= 1;
        return *this;
    }
    constexpr mint &operator/=(mint rhs) noexcept {
        u64 exp=get_mod()-2;
        while (exp) {
            if (exp % 2) {
                *this *= rhs;
            }
            rhs *= rhs;
            exp /= 2;
        }
        return *this;
    }
    constexpr bool operator==(mint x) noexcept {
        return a==x.a;
    }
    constexpr bool operator!=(mint x) noexcept {
        return a!=x.a;
    }
    constexpr bool operator<(mint x) noexcept {
        return a<x.a;
    }
    constexpr bool operator>(mint x) noexcept {
        return a>x.a;
    }
    constexpr bool operator<=(mint x) noexcept {
        return a<=x.a;
    }
    constexpr bool operator>=(mint x) noexcept {
        return a>=x.a;
    }
    constexpr static int root(){
        mint root = 2;
        while(root.pow((get_mod()-1)>>1).a==1)root++;
        return root.a;
    }
    constexpr mint pow(long long n){
        long long x=a;
        mint ret = 1;
        while(n>0) {
            if(n&1)(ret*=x);
            (x*=x)%=get_mod();
            n>>=1;
        }
        return ret;
    }
    constexpr mint inv(){
        return pow(get_mod()-2);
    }
    static vector<mint> fac,ifac;
    static bool init;
    constexpr static int mx=10000001;
    void build(){
        init=0;
        fac.resize(mx);
        ifac.resize(mx);
        fac[0]=1,ifac[0]=1;
        for(int i=1;i<mx;i++)fac[i]=fac[i-1]*i;
        ifac[mx-1]=fac[mx-1].inv();
        for(int i=mx-2;i>=0;i--)ifac[i]=ifac[i+1]*(i+1);
    }
    mint comb(long long b){
        if(init)build();
        if(a==0&&b==0)return 1;
        if((long long)a<b)return 0;
        return fac[a]*ifac[a-b]*ifac[b];
    }
    mint fact(){
        if(init)build();
        return fac[a];
    }
    mint fact_inv(){
        if(init)build();
        return ifac[a];
    }
    friend ostream& operator<<(ostream& lhs, const mint& rhs) noexcept {
        lhs << rhs.a;
        return lhs;
    }
    friend istream& operator>>(istream& lhs,mint& rhs) noexcept {
        lhs >> rhs.a;
        return lhs;
    }
    constexpr static u64 get_mod(){return MOD;}
};
vector<mint> mint::fac;
vector<mint> mint::ifac;
bool mint::init=1;
#line 1 "cpplib/data_structure/dsu/union_find.hpp"
/**
 * @brief Union Find
 * @docs docs/union_find.md
 * @see https://ja.wikipedia.org/wiki/%E7%B4%A0%E9%9B%86%E5%90%88%E3%83%87%E3%83%BC%E3%82%BF%E6%A7%8B%E9%80%A0
 */

class UF{
    public:
    vector<int> data;
    int sz;
	public:
    UF(int sz):sz(sz){data.resize(sz,-1);}
    bool unite(int x,int y){
        x=root(x);y=root(y);
        if(x==y)return 0;
        if(data[x]>data[y])swap(x,y);
		data[x]+=data[y];
		data[y]=x;
		sz--;
        return 1;
    }
    inline int root(int x){return data[x]<0?x:data[x]=root(data[x]);}
    inline bool same(int x, int y){return root(x)==root(y);}
    inline int size(){return sz;}
	inline int size(int x){return -data[root(x)];}
};
#line 8 "code.cpp"

int main(){
    lint n,c;
    cin>>n>>c;
    mat g(n);
    rep(i,n-1){
        lint s,t;
        cin>>s>>t;
        s--;t--;
        g[s].push_back(t);
        g[t].push_back(s);
    }
    auto memo=make_vector<lint>(-1,n,min(8*n,c)+1);
    auto f=[&](auto f,lint now,lint p,lint val)->lint{
        if(memo[now][val]!=-1)return memo[now][val];
        mint ans=1;
        for(lint e:g[now]){
            if(p==e)continue;
            mint tmp=0;
            if(val-3>=0)tmp+=f(f,e,now,val-3);
            if(val+3<c)tmp+=f(f,e,now,val+3);
            ans*=tmp;
        }
        return memo[now][val]=ans.a;
    };
    if(4*n>=c){
        mint ans2=0;
        rep(i,c){
            ans2+=f(f,0,-1,i);
        }
        cout<<ans2<<endl;
        return 0;
    }
    vec ans(min(4*n,c));
    rep(i,min(4*n,c)){
        ans[i]=f(f,0,-1,i);
    }
    mint ans2=0;
    set<lint>used;
    rep(i,4*n){
        used.insert(i);
        ans2+=ans[min(i,c-i)];
    }
    rep(i,4*n){
        if(used.count(c-i))continue;
        used.insert(c-i);
        ans2+=ans[min(c-i,i)];
    }
    cout<<ans2+mint(c-used.size())*ans.back()<<endl;
}