#include <bits/stdc++.h>
using namespace std;
#define POPCOUNT(n) (__builtin_popcountll((n)))
#define CLZ(n) (__builtin_clzll((n)))
#define CTZ(n) (__builtin_ctzll((n)))
#define LOG(n) (63 - __builtin_clzll((n)))
#define BIT(n, i) (((n) >> (i)) & 1ll)
#define MASK(i) (1ll << (i))
#define FLIP(n, i) ((n) ^ (1ll << (i)))
#define ON(n, i) ((n) | MASK(i))
#define OFF(n, i) ((n) & ~MASK(i))
#define Int __int128
#define fi first
#define se second
typedef long long ll;
typedef unsigned long long ull;
typedef long double ld;
typedef pair<int, int> pii;
typedef pair<long long, long long> pll;
typedef pair<long long, int> pli;
typedef pair<int, long long> pil;
typedef vector<pair<int, int>> vii;
typedef vector<pair<long long, long long>> vll;
typedef vector<pair<long long, int>> vli;
typedef vector<pair<int, long long>> vil;
template <class T1, class T2> bool maximize(T1 &x, T2 y) {
if (x < y) {
x = y;
return true;
}
return false;
}
template <class T1, class T2> bool minimize(T1 &x, T2 y) {
if (x > y) {
x = y;
return true;
}
return false;
}
template <class T> void remove_duplicate(vector<T> &ve) {
sort (ve.begin(), ve.end());
ve.resize(unique(ve.begin(), ve.end()) - ve.begin());
}
mt19937_64 rng(chrono::high_resolution_clock::now().time_since_epoch().count());
long long random(long long l, long long r) {
return uniform_int_distribution<long long>(l, r)(rng);
}
unsigned long long random(unsigned long long l, unsigned long long r) {
return uniform_int_distribution<unsigned long long>(l, r)(rng);
}
template <class T> T random(T r) {
return rng() % r;
}
const int N = 1e6 + 5;
const int MOD = 1e9 + 7;
const int inf = 1e9;
const long long INF = 1e18;
int n;
ll D;
int f[N], g[N], cnt[N], par[N];
vector<int> adj[N];
void dfs(int u, int fa) {
for (auto v : adj[u]) if (v != fa) {
par[v] = u, dfs(v, u);
cnt[u] += f[v];
}
f[u] = (cnt[u] == 0);
}
void reroot(int u, int fa) {
for (auto v : adj[u]) if (v != fa) {
int cur = g[u] + (cnt[u] - f[v]);
g[v] = (cur == 0);
reroot(v, u);
}
}
int binpow(int x, ll y) {
int ans = 1;
while (y > 0) {
if (y & 1) ans = 1ll * ans * x % MOD;
x = 1ll * x * x % MOD, y >>= 1;
}
return ans;
}
signed main() {
ios_base::sync_with_stdio(false); cin.tie(NULL); cout.tie(NULL);
cin >> n >> D;
for (int i = 1; i < n; ++i) {
int u, v; cin >> u >> v;
adj[u].emplace_back(v);
adj[v].emplace_back(u);
}
dfs(1, -1);
g[1] = 0; reroot(1, -1);
int init = 0, lose = 0;
for (int i = 1; i <= n; ++i) {
init += f[i] == f[1];
if (f[1]) init -= f[i] && f[par[i]];
f[i] &= !g[i];
if (f[i]) ++lose;
}
int ans = init; ll pw = D / 2;
cout << 1ll * ans * binpow(n, pw) % MOD * binpow(lose, D - pw) % MOD;
return 0;
}
