#include <algorithm>
#include <climits>
#include <cmath>
#include <cstdio>
#include <cstdlib>
#include <ctime>
#include <iostream>
#include <sstream>
#include <functional>
#include <map>
#include <string>
#include <cstring>
#include <vector>
#include <queue>
#include <stack>
#include <deque>
#include <set>
#include <list>
#include <numeric>
using namespace std;
const double PI = 3.14159265358979323846;
const double EPS = 1e-12;
const int INF = 1<<25;
typedef pair<int,int> P;
typedef long long ll;
typedef unsigned long long ull;
#define MAX_N 2000
int N, K;
vector<int> g[MAX_N];
ll dp[MAX_N][MAX_N];
ll mod = 1000000007;

ll dfs(int u){
	dp[u][0] = 1;
	ll sz = 1;
	for(int i = 0; i < g[u].size(); i++){
		int v = g[u][i];
		ll t = dfs(v);
		sz += t;
		for(int j = sz-1; j >= 0; j--){
			for(int k = 1; k <= (min<int>(t, j)); k++){
				(dp[u][j]+=dp[u][j-k]*dp[v][k])%=mod;
			}
		}
	}
	dp[u][sz] = 1;
	return sz;
}

int main(){
	cin>>N>>K;
	for(int i = 0; i < N-1; i++){
		int a, b;
		cin>>a>>b;
		g[a].push_back(b);
	}
	dfs(0);
	cout<<dp[0][K]<<endl;
	return 0;
}