B-B'6

#include <bits/stdc++.h>
using namespace std;
#define all(a) a.begin(), a.end()
#define forn(i, a, b) for (ll i = a; i < b; i++)
#define FAST ios_base::sync_with_stdio(false); cin.tie(nullptr)
using ll = long long;
vector< vector<int> > node;
vector<bool> used;
vector<int> cnt, ans, cnt_edges;
int n, s;
const int MOD = 1e9 + 7;

void dfs(int curr) {
    used[curr] = true;
    ans[curr] = max(s - cnt[curr], 0);
    bool isfirst = true;
    for (auto next : node[curr]) {
        if (used[next]) continue;
        if (isfirst)
            cnt[next] = min(2, cnt[curr] + 1);
        else
            cnt[next] = cnt_edges[curr] + 1;
        cnt_edges[next] += 1;
        cnt_edges[curr] += 1;
        dfs(next);
        if (isfirst) isfirst = false;
    }
}

int main() {
    FAST;
    cin >> n >> s;
    node.resize(n);
    used.resize(n);
    cnt_edges.resize(n);
    cnt.resize(n);
    ans.resize(n);
    forn(i, 1, n) {
        int u, v;
        cin >> u >> v;
        u--; v--;
        node[u].push_back(v);
        node[v].push_back(u);
    }
    dfs(0);
    ll answer = 1;
    for (auto e : ans) answer = (answer * e) % MOD;
    //answer = (answer * cnk(n, count(all(ans), 0))) % MOD;
    //I EXPECT THERE IS POSSIBLE ZEROES BUT IN THIS TASK IT WASN'T POSSIBLE LOL!
    cout << answer << endl;
}