#include <bits/stdc++.h>
using namespace std;
typedef long long ll;
const int N = 100000, mod = 1e9 + 7;
int add(int a, int b) {
return a + b < mod ? a + b : a + b - mod;
}
int mul(int a, int b) {
return a * (ll) b % mod;
}
int binp(int a, ll p) {
int ans = 1;
for (; p > 0; p >>= 1, a = mul(a, a)) {
if (p & 1) {
ans = mul(ans, a);
}
}
return ans;
}
int sub(int a, int b) {
return a >= b ? a - b : a - b + mod;
}
int inv(int a) {
return binp(a, mod - 2);
}
vector<int> g[N];
int main() {
ios::sync_with_stdio(false);
cin.tie(nullptr);
int n;
ll d;
cin >> n >> d;
for (int i = 1; i < n; ++i) {
int a, b;
cin >> a >> b;
--a, --b;
g[a].push_back(b);
g[b].push_back(a);
}
vector<bool> dp(n);
vector<int> cntLoosing(n), c(n), total(n), only_loosing(n);
function<void(int, int)> dfs1 = [&](int v, int p) {
dp[v] = false;
cntLoosing[v] = 0;
for (int to: g[v]) {
if (to != p) {
dfs1(to, v);
if (!dp[to]) {
only_loosing[v] += c[to];
++cntLoosing[v];
dp[v] = true;
}
total[v] += c[to];
}
}
if (cntLoosing[v] > 1) {
c[v] = 0;
} else if (cntLoosing[v] == 1) {
c[v] = only_loosing[v];
} else {
c[v] = total[v] + 1;
}
};
function<void(int, int)> dfs2 = [&](int v, int p) {
dp[v] = cntLoosing[v];
if (cntLoosing[v] > 1) {
c[v] = 0;
} else if (cntLoosing[v] == 1) {
c[v] = only_loosing[v];
} else {
c[v] = total[v] + 1;
}
for (int to: g[v]) {
if (to != p) {
int gg = c[to];
total[v] -= gg;
bool zz = dp[to];
if (!dp[to]) {
--cntLoosing[v];
only_loosing[v] -= gg;
}
int now;
if (cntLoosing[v] > 1) {
now = 0;
} else if (cntLoosing[v] == 1) {
now = only_loosing[v];
} else {
now = total[v] + 1;
dp[to] = true;
++cntLoosing[to];
only_loosing[to] += now;
}
total[to] += now;
dfs2(to, v);
total[v] += gg;
if (!zz) {
cntLoosing[v] += 1;
only_loosing[v] += gg;
}
}
}
};
dfs1(0, -1);
dfs2(0, -1);
int E = 0, L = 0;
for (int i = 0; i < n; ++i) {
if (dp[i]) {
E = add(E, c[i]);
} else {
L += 1;
E = sub(E, c[i]);
}
}
int a = mul(n, n);
int x = mul(L, mul(sub(binp(a, d), binp(E, d)), inv(sub(a, E))));
int ans;
if (dp[0]) {
ans = mul(x, c[0]);
} else {
ans = sub(binp(a, d), mul(x, c[0]));
}
cout << sub(binp(a, d), ans);
return 0;
}
