#include <iostream>
#include <vector>
using namespace std;
const int MAXN = 100000;
const int MOD = 1000000007;
vector<int> g[MAXN + 1];
int n, sz[MAXN + 1], down[MAXN + 1], down_move[MAXN + 1];
int cnt, can_win[MAXN + 1], win_move[MAXN + 1], p[MAXN + 1];
void subSelf(int &a, int b) {
a -= b;
if(a < 0) {
a += MOD;
}
}
void addSelf(int &a, int b) {
a += b;
if(a >= MOD) {
a -= MOD;
}
}
struct Rec {
int a, b; // a * N^(2*d-1) + b * win_cnt
void operator -=(const Rec &other) {
subSelf(a, other.a);
subSelf(b, other.b);
}
void operator +=(const Rec &other) {
addSelf(a, other.a);
addSelf(b, other.b);
}
} rec_down[MAXN + 1], rec_up[MAXN + 1], rec_win[MAXN + 1], sum_sons[MAXN + 1], next_cnt;
struct Matrix {
int val[2][2];
};
Matrix operator *(Matrix a, Matrix b) {
Matrix c;
for(int i = 0; i < 2; i++) {
for(int j = 0; j < 2; j++) {
c.val[i][j] = 0;
for(int k = 0; k < 2; k++) {
c.val[i][j] = (c.val[i][j] + 1LL * a.val[i][k] * b.val[k][j]) % MOD;
}
}
}
return c;
}
void dfs(int node, int parent) {
p[node] = parent;
sz[node] = 1;
for(int son : g[node]) {
if(son != parent) {
dfs(son, node);
sum_sons[node] += rec_down[son];
sz[node] += sz[son];
if(down[son] == 0) {
down[node]++;
down_move[node] = son;
}
}
}
rec_down[node].a = sz[node];
if(down[node] == 0) {
rec_down[node].b = MOD - 1;
rec_down[node] -= sum_sons[node];
} else if(down[node] == 1) {
rec_down[node] -= rec_down[down_move[node]];
}
}
void dfs2(int node, int parent) {
can_win[node] = down[node];
win_move[node] = down_move[node];
if(node > 1 && (can_win[parent] == 0 || (can_win[parent] == 1 && win_move[parent] == node))) {
can_win[node]++;
win_move[node] = parent;
}
if(can_win[node] > 0) {
cnt++;
}
for(int son : g[node]) {
if(son != parent) {
dfs2(son, node);
}
}
}
void dfs3(int node, int parent) {
if(node > 1) {
rec_up[node].a = n - sz[node];
if(can_win[parent] == 0) {
rec_up[node].b = MOD - 1;
rec_up[node] -= rec_up[parent];
rec_up[node] -= sum_sons[parent];
rec_up[node] += rec_down[node];
} else if(can_win[parent] == 1) {
if(win_move[parent] == p[parent]) {
rec_up[node] -= rec_up[parent];
} else {
rec_up[node] -= rec_down[win_move[parent]];
}
}
}
rec_win[node].a = n;
if(can_win[node] == 0) {
rec_win[node].b = MOD - 1;
rec_win[node] -= rec_up[parent];
rec_win[node] -= sum_sons[node];
} else if(can_win[node] == 1) {
if(win_move[node] == parent) {
rec_win[node] -= rec_up[parent];
} else {
rec_win[node] -= rec_down[win_move[node]];
}
}
next_cnt += rec_win[node];
for(int son : g[node]) {
if(son != parent) {
dfs3(son, node);
}
}
}
int main() {
ios_base::sync_with_stdio(0);
cin.tie(0);
long long d;
cin >> n >> d;
for(int i = 1; i < n; i++) {
int u, v;
cin >> u >> v;
g[u].push_back(v);
g[v].push_back(u);
}
dfs(1, 0);
dfs2(1, 0);
dfs3(1, 0);
Matrix aux;
aux.val[0][0] = n;
aux.val[0][1] = 0;
aux.val[1][0] = 0;
aux.val[1][1] = cnt;
Matrix rec;
rec.val[0][0] = 1LL * n * n % MOD;
rec.val[0][1] = next_cnt.a;
rec.val[1][0] = 0;
rec.val[1][1] = next_cnt.b;
d--;
while(d > 0) {
if(d & 1) {
aux = aux * rec;
}
rec = rec * rec;
d >>= 1;
}
cout << (1LL * aux.val[0][0] * rec_win[1].a + 1LL * aux.val[1][1] * rec_win[1].b) % MOD << "\n";
return 0;
}
