#include <bits/stdc++.h>
using namespace std;
const long long mod = 1e9 + 7;
const int MaxN = 1e6 + 5;
int n;
long long d;
vector<int> graph[MaxN];
void Inp()
{
cin >> n >> d;
for (int x = 1; x < n; x++)
{
int u, v;
cin >> u >> v;
graph[u].push_back(v);
graph[v].push_back(u);
}
}
struct Matrix
{
vector<vector<long long>> mat;
Matrix() = default;
Matrix(vector<vector<long long>> _mat)
{
mat = _mat;
}
Matrix& operator = (const Matrix& other)
{
this->mat = other.mat;
return *this;
}
Matrix operator + (const Matrix& other) const
{
Matrix res;
assert((int)this->mat.size() == (int)other.mat.size());
res.mat.resize((int)this->mat.size());
for (int x = 0; x < (int)this->mat.size(); x++)
{
assert((int)this->mat[x].size() == (int)other.mat[x].size());
res.mat[x].resize((int)other.mat[x].size());
for (int y = 0; y < (int)other.mat[x].size(); y++)
{
res.mat[x][y] = (this->mat[x][y] + other.mat[x][y]) % mod;
}
}
return res;
}
Matrix operator - (const Matrix& other) const
{
Matrix res;
assert((int)this->mat.size() == (int)other.mat.size());
res.mat.resize((int)this->mat.size());
for (int x = 0; x < (int)this->mat.size(); x++)
{
assert((int)this->mat[x].size() == (int)other.mat[x].size());
res.mat[x].resize((int)other.mat[x].size());
for (int y = 0; y < (int)other.mat[x].size(); y++)
{
res.mat[x][y] = (this->mat[x][y] - other.mat[x][y]) % mod;
}
}
return res;
}
Matrix operator * (const Matrix& other) const
{
Matrix res;
assert((int)this->mat[0].size() == (int)other.mat.size());
res.mat.resize((int)this->mat.size());
for (int x = 0; x < (int)res.mat.size(); x++)
{
res.mat[x].resize((int)other.mat[x].size());
for (int y = 0; y < (int)res.mat[x].size(); y++)
{
res.mat[x][y] = 0;
for (int z = 0; z < (int)this->mat[0].size(); z++)
{
res.mat[x][y] = (res.mat[x][y] + this->mat[x][z] * other.mat[z][y] % mod) % mod;
}
}
}
return res;
}
void Print()
{
for (int x = 0; x < (int)mat.size(); x++)
{
for (int y = 0; y < (int)mat[x].size(); y++)
{
cout << mat[x][y] << " ";
}
cout << "\n";
}
cout << "\n";
}
};
Matrix F[MaxN];
bool isW[MaxN];
int cntnow[MaxN];
void DFS(int u, int p)
{
isW[u] = false;
int cnt = 0;
for (int v : graph[u])
{
if (v != p)
{
DFS(v, u);
isW[u] |= !isW[v];
cnt += !isW[v];
}
}
cntnow[u] = cnt;
F[u] = Matrix({{isW[u], 1}, {!isW[u], 0}});
for (int v : graph[u])
{
if (v != p)
{
if (cnt == !isW[v])
{
F[u] = (F[u] + (Matrix({{0, 1}, {1, 0}}) * F[v]));
}
else
{
F[u] = (F[u] + (Matrix({{1, 1}, {0, 0}}) * F[v]));
}
}
}
}
bool Wroot[MaxN];
Matrix PreRoot[MaxN];
void Exc()
{
for (int x = 1; x <= n; x++)
{
DFS(x, -1);
Wroot[x] = isW[x];
PreRoot[x] = F[x];
}
Matrix change({{0, 0}, {0, 0}});
Matrix sam({{0}, {0}});
for (int x = 1; x <= n; x++)
{
sam.mat[!Wroot[x]][0]++;
change = (change + PreRoot[x]);
}
if (d > 1)
{
d -= 2;
sam = change * sam;
while (d)
{
if (d & 1)
{
sam = change * sam;
}
change = change * change;
d >>= 1;
}
}
DFS(1, -1);
long long res = (F[1].mat[0][0] * sam.mat[0][0] % mod + F[1].mat[0][1] * sam.mat[1][0] % mod) % mod;
cout << (res % mod + mod) % mod;
}
int main()
{
//freopen("C.INP", "r", stdin);
//freopen("C.ANS", "w", stdout);
ios_base::sync_with_stdio(false);
cin.tie(nullptr);
int test = 1;
//cin >> test;
for (int x = 1; x <= test; x++)
{
Inp();
Exc();
}
return 0;
}
