#pragma GCC target("avx2")
#pragma GCC optimize("O3")
#pragma GCC optimize("unroll-loops")

#include <algorithm>
#include <bitset>
#include <climits>
#include <cmath>
#include <cstring>
#include <deque>
#include <forward_list>
#include <iomanip>
#include <iostream>
#include <list>
#include <map>
#include <queue>
#include <set>
#include <string>
#include <unordered_map>
#include <unordered_set>
#include <utility>
#include <vector>

using namespace std;
typedef long long ll;
typedef pair<int, int> P;
typedef bitset<16> BS;

const ll MOD = 1E+09 + 7;
const ll INF = 1E18;
const int MAX_N = 1000;

ll N, C;

ll dp[MAX_N + 1][6 * MAX_N];
vector<ll> G[MAX_N];

ll tree(ll i, ll par, ll j);
ll modPow(ll x, ll a);

int main()
{
    ios::sync_with_stdio(false);
    cin.tie(nullptr);

    cin >> N >> C;

    for (int i = 1; i <= N - 1; i++) {
        ll a, b;
        cin >> a >> b;
        G[a].push_back(b);
        G[b].push_back(a);
    }

    fill(dp[0], dp[N + 1], -1);
    ll ans = 0;
    for (int i = 1; i <= min(C, 6 * N - 5); i++) {
        ans += tree(1, 0, i);
        ans %= MOD;
    }
    ans += (modPow(2, N - 1) * max(0LL, C - 6 * N + 5)) % MOD;
    ans %= MOD;

    /*
    for (int i = 0; i < N; i++) {
        for (int j = 0; j < N; j++) {
            cout << "i = " << i << ", j = " << j << ", dp = " << dp[i][j] << "\n";
        }
    }
    */

    cout << ans << "\n";

    return 0;
}

ll tree(ll i, ll par, ll j)
{
    if (j < 1 || j > min(C, 6 * N - 5))
        return 0;
    if (dp[i][j] != -1)
        return dp[i][j];
    ll res = 1;
    for (auto c : G[i]) {
        if (c != par) {
            res *= (tree(c, i, j + 3) + tree(c, i, j - 3)) % MOD;
            res %= MOD;
        }
    }
    return dp[i][j] = res;
}

ll modPow(ll x, ll a)
{
    ll ans = 1;
    while (a > 0) {
        if (a & 1) {
            ans = (ans * x) % MOD;
        }
        x = (x * x) % MOD;
        a >>= 1;
    }
    return ans;
}