#include<bits/stdc++.h>
using namespace std;
using LL = long long;
using ULL = unsigned long long;
#define rep(i,n) for(int i=0; i<(n); i++)

const ULL M = 1000000007;

int N;
int C;
vector<int> E[1000];
int P[1000];

ULL dp[1000][3001] = {};

void dfs(int p) {
    rep(i, min((C + 1) / 2, 3001)) dp[p][i] = 1;
    for (int e : E[p]) {
        if (P[p] == e) continue;
        P[e] = p;
        dfs(e);
        rep(i, min((C + 1) / 2, 3)) dp[p][i] = dp[p][i] * dp[e][min(min(C - i - 4, i + 3), 3000)] % M;
        for (int i = 3; i < min((C + 1) / 2, 3000); i++)
            dp[p][i] = dp[p][i] * (dp[e][i - 3] + dp[e][min(min(C - i - 4, i + 3), 3000)]) % M;
        dp[p][3000] = dp[p][3000] * dp[e][3000] * 2 % M;
    }
}

int main() {
    cin >> N >> C;
    rep(i, N - 1) {
        int u, v; cin >> u >> v; u--; v--;
        E[u].push_back(v);
        E[v].push_back(u);
    }

    rep(i, N) P[i] = -1;

    dfs(0);

    ULL ans = 0;
    rep(i, min(C / 2, 3000)) ans += dp[0][i] * 2;
    if (C <= 5999) if (C % 2 == 1) ans += dp[0][C / 2];
    if (C >= 6000) ans += dp[0][3000] * (C - 6000) % M;
    ans %= M;

    cout << ans << endl;

    return 0;
}