/* -*- coding: utf-8 -*-
 *
 * 1227.cc:  No.1227 I hate ThREE - yukicoder
 */

#include<cstdio>
#include<cstdlib>
#include<cstring>
#include<cmath>
#include<iostream>
#include<string>
#include<vector>
#include<map>
#include<set>
#include<stack>
#include<list>
#include<queue>
#include<deque>
#include<algorithm>
#include<numeric>
#include<utility>
#include<complex>
#include<functional>
 
using namespace std;

/* constant */

const int MAX_N = 1000;
const int MOD = 1000000007;

/* typedef */

typedef long long ll;
typedef vector<int> vi;
typedef queue<int> qi;

/* global variables */

vi nbrs[MAX_N];
int ps[MAX_N], cns[MAX_N];
int dp[MAX_N][MAX_N * 6];

/* subroutines */

inline void addmod(int &a, int b) { a = (a + b) % MOD; }
inline void mulmod(int &a, int b) { a = (ll)a * b % MOD; }

/* main */

int main() {
  int n, c;
  scanf("%d%d", &n, &c);

  for (int i = 1; i < n; i++) {
    int a, b;
    scanf("%d%d", &a, &b);
    a--, b--;
    nbrs[a].push_back(b);
    nbrs[b].push_back(a);
  }

  ps[0] = -1;
  qi q;
  q.push(0);

  while (! q.empty()) {
    int u = q.front(); q.pop();
    int up = ps[u];
    vi &nbru = nbrs[u];

    for (vi::iterator vit = nbru.begin(); vit != nbru.end(); vit++) {
      int v = *vit;
      if (v != up) {
	ps[v] = u;
	cns[u]++;
	q.push(v);
      }
    }
  }

  int m = min(c, n * 6 - 5);

  for (int u = 0; u < n; u++) {
    fill(dp[u], dp[u] + m, 1);
    if (cns[u] == 0) q.push(u);
  }

  while (! q.empty()) {
    int u = q.front(); q.pop();
    int up = ps[u];

    if (up >= 0) {
      for (int i = 0; i < m; i++) {
	int d = 0;
	if (i >= 3) addmod(d, dp[u][i - 3]);
	if (i + 3 < m) addmod(d, dp[u][i + 3]);
	mulmod(dp[up][i], d);
      }

      if (--cns[up] == 0) q.push(up);
    }
  }

  int sum = 0;
  for (int i = 0; i < m; i++) addmod(sum, dp[0][i]);

  if (m < c) addmod(sum, (ll)dp[0][n * 3 - 2] * (c - m) % MOD);

  printf("%d\n", sum);
  return 0;
}