#include<bits/stdc++.h>
using namespace std;
#define fs first
#define sc second
#define pb push_back
#define mp make_pair
#define eb emplace_back
#define ALL(A) A.begin(),A.end()
#define RALL(A) A.rbegin(),A.rend()
typedef long long LL;
typedef pair<LL,LL> P;
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; }
template<typename T> T gcd(T a,T b){return b?gcd(b,a%b):a;}
const LL mod=1000000007;
const LL LINF=1LL<<60;
const int INF=1<<30;
int dx[]={1,0,-1,0,1,-1,1,-1};
int dy[]={0,1,0,-1,1,-1,-1,1};

LL n,c;
vector<int> G[1<<10];
vector<vector<LL>> dp(1<<10, vector<LL> (10000, -1));

LL dfs(int u,int v,int s){
    if(~dp[u][s]) return dp[u][s];
    LL ret = 1;
    for(auto g:G[u]){
        if(g == v) continue;
        LL tmp = 0;
        if(s + 3 < c) tmp = (tmp + dfs(g, u, s + 3))%mod;
        if(s - 3 >= 0) tmp = (tmp + dfs(g, u, s - 3))%mod;
        ret = (ret * tmp)%mod;
    }
    return dp[u][s] = ret;
}

LL fpow(LL a, LL n){
    LL x = 1;
    while(n > 0){
        if(n&1){
            x=x*a%mod;
        }
        a=a*a%mod;
        n >>= 1;
    }
    return x;
}
int main(){
    cin >> n >> c;
    for (int i = 0; i < n-1; i++) {
        int a,b;cin >> a >> b;
        a--,b--;
        G[a].pb(b);
        G[b].pb(a);
    }
    LL ans = 0;
    if(c <= 8000){
        for (int i = 0; i < c; i++) {
            ans = (ans + dfs(0, -1, i))%mod;
        }
    }
    else{
        for (int i = 0; i < 4000; i++) {
            ans = (ans + 2LL * dfs(0, -1, i))%mod;
        }
        ans = (ans + (c - 8000) * dp[0][3999])%mod;
    }
    cout << ans << endl;
}