#include <bits/stdc++.h>

const int maxn = 1e3 + 3;
using ll = long long;
const ll mo = 1e9 + 7;

int n, C, p[maxn];
std::vector<int> E[maxn];
ll dp[maxn][maxn * 3 * 2 + 3];

void dfs(int u) {
    for(int i = 1; i <= C && i <= n * 3 * 2 + 2; ++i) {
        dp[u][i] = 1;
    }
    for(auto &&v : E[u]) {
        if(v == p[u])
            continue;
        p[v] = u;
        dfs(v);
        for(int i = 1; i <= C && i <= n * 3 * 2 + 2; ++i) {
            ll wu = (i - 3 >= 1) ? dp[v][i - 3] : 0ll;
            ll wv = (i + 3 <= C && i + 3 <= n * 3 * 2 + 2) ? dp[v][i + 3] : 0ll;
            dp[u][i] = dp[u][i] * ((wu + wv) % mo) % mo;
        }
    }
}

int main() {
    scanf("%d%d", &n, &C);
    for(int i = 0; i < n - 1; ++i) {
        int u, v;
        scanf("%d%d", &u, &v);
        E[u].push_back(v);
        E[v].push_back(u);
    }
    dfs(1);
    ll ans = 0;
    if(C > n * 3 * 2 + 2) {
        for(int i = 1; i <= n * 3; ++i) {
            ans = (ans + dp[1][i]) % mo;
        }
        for(int i = 1; i <= n * 3; ++i) {
            ans = (ans + dp[1][n * 3 * 2 + 2 - i + 1]) % mo;
        }
        ans = (ans + dp[1][n * 3 + 1] * (C - n * 3 * 2) % mo) % mo;
    } else {
        for(int i = 1; i <= C; ++i) {
            ans = (ans + dp[1][i]) % mo;
        }
    }
    printf("%lld\n", (ans % mo + mo) % mo);
    return 0;
}