#include <bits/stdc++.h>
#define ll long long
#define pii pair<int, int>
#define pll pair<ll, ll>
#define F first
#define S second
using namespace std;
const ll MOD = 1000000007;
ll N, D, dp[100005], pre[100005], subtree[100005], crit[100005], L, K, dpL[100005], Npow[100005];
vector<int> E[100005];
void dfs(int k)
{
for (int i : E[k])
if (i != pre[k])
{
pre[i] = k;
dfs(i);
if (dp[i] == 0)
dp[k]++;
}
}
void dfs2(int k, bool w)
{
//cout << "dfs2 " << k << '\n';
if (w && dp[k] == 1)
for (int i : E[k])
{
if (i != pre[k])
{
if (dp[i] == 0)
{
dfs2(i, false);
subtree[k] += subtree[i];
}
else
dfs2(i, true);
}
}
else if (w)
for (int i : E[k])
{
if (i != pre[k])
dfs2(i, true);
}
else if (!w)
{
subtree[k]++;
for (int i : E[k])
if (i != pre[k])
{
dfs2(i, true);
subtree[k] += subtree[i];
}
}
}
void reroot(int k)
{
//cout << "reroot " << k << '\n';
//for (int i = 1; i <= N; i++)
// cout << subtree[i] << " \n"[i == N];
//for (int i = 1; i <= N; i++)
// cout << dp[i] << " \n"[i == N];
int sum = 0;
if (dp[k] == 0)
crit[k] = 1;
for (int i : E[k])
{
if (dp[k] == 1 && dp[i] == 0)
crit[k] += subtree[i];
else if (dp[k] == 0)
crit[k] += subtree[i];
if (dp[k] == 2 && dp[i] == 0)
sum += subtree[i];
else if (dp[k] == 1 && dp[i] == 1)
sum += subtree[i];
}
swap(subtree[k], crit[k]);
for (int i : E[k])
if (i != pre[k])
{
if (dp[k] == 1 && dp[i] == 0)
{
ll tmp = sum + 1;
swap(tmp, subtree[k]);
dp[k]--, dp[i]++;
reroot(i);
dp[k]++;
swap(tmp, subtree[k]);
}
else if (dp[k] >= 2 && dp[i] == 0)
{
ll tmp = (dp[k] == 2 ? sum - subtree[i] : 0);
swap(tmp, subtree[k]);
dp[k]--;
reroot(i);
dp[k]++;
swap(tmp, subtree[k]);
}
else if (dp[k] == 0)
{
ll tmp = subtree[k] - subtree[i];
swap(tmp, subtree[k]);
dp[i]++;
reroot(i);
swap(tmp, subtree[k]);
}
else
reroot(i);
}
swap(subtree[k], crit[k]);
}
signed main()
{
cin >> N >> D;
for (int i = 1; i < N; i++)
{
int u, v;
cin >> u >> v;
E[u].emplace_back(v);
E[v].emplace_back(u);
}
dfs(1);
dfs2(1, dp[1] != 0);
reroot(1);
//for (int i = 1; i <= N; i++)
// cout << crit[i] << " \n"[i == N];
//for (int i = 1; i <= N; i++)
// cout << dp[i] << " \n"[i == N];
for (int i = 1; i <= N; i++)
if (dp[i] == 0)
L++, K = (K - crit[i] + MOD) % MOD;
else
K = (K + crit[i] + MOD) % MOD;
dpL[0] = L;
Npow[0] = 1;
for (int i = 1; i <= D; i++)
{
Npow[i] = (Npow[i - 1] * N % MOD) * N % MOD;
dpL[i] = ((L * Npow[i] % MOD) + (K * dpL[i - 1])) % MOD;
}
if (dp[1] == 0)
cout << (dpL[D - 1] * crit[1]) % MOD << '\n';
else
cout << (Npow[D] - (dpL[D - 1] * crit[1]) % MOD + MOD) % MOD << '\n';
}
/*
7 1
1 2
2 3
3 4
4 5
5 6
6 7
*/
