#include <bits/stdc++.h>
#include <ext/pb_ds/assoc_container.hpp>
#include <ext/pb_ds/tree_policy.hpp>
#define ll long long
#define ld long double
#define ull unsigned long long
#define ff first
#define ss second
#define pii pair<int,int>
#define pll pair<long long, long long>
#define vi vector<int>
#define vl vector<long long>
#define pb push_back
#define rep(i, b) for(int i = 0; i < (b); ++i)
#define rep2(i,a,b) for(int i = a; i <= (b); ++i)
#define rep3(i,a,b,c) for(int i = a; i <= (b); i+=c)
#define count_bits(x) __builtin_popcountll((x))
#define all(x) (x).begin(),(x).end()
#define siz(x) (int)(x).size()
#define forall(it,x) for(auto& it:(x))
using namespace __gnu_pbds;
using namespace std;
typedef tree<int, null_type, less<int>, rb_tree_tag,tree_order_statistics_node_update> ordered_set;
//mt19937 mt;void random_start(){mt.seed(chrono::time_point_cast<chrono::milliseconds>(chrono::high_resolution_clock::now()).time_since_epoch().count());}
//ll los(ll a, ll b) {return a + (mt() % (b-a+1));}
const int INF = 1e9+50;
const ll INF_L = 1e18+40;
const ll MOD = 1e9+7;
struct node
{
ll dp[2][2];
int sub_siz = 0;
bool is_win = 0;
};
struct dp_
{
ll dp[2][2];
dp_()
{
rep(d1,2) rep(d2,2) dp[d1][d2] = 0;
}
dp_ operator+(const dp_& other)
{
dp_ res;
res.dp[0][0] = other.dp[0][0] * dp[0][0] + other.dp[1][0] * dp[0][1];
res.dp[0][1] = other.dp[0][1] * dp[0][0] + other.dp[1][1] * dp[0][1];
res.dp[1][0] = other.dp[0][0] * dp[1][0] + other.dp[1][0] * dp[1][1];
res.dp[1][1] = other.dp[0][1] * dp[1][0] + other.dp[1][1] * dp[1][1];
rep(d1,2) rep(d2,2) res.dp[d1][d2] %= MOD;
return res;
}
};
vi graph[100001];
node dp_node[100001];
void dfs1(int v, int pop)
{
dp_node[v].sub_siz = 1;
dp_node[v].is_win = 0;
rep(d1,2) rep(d2,2) dp_node[v].dp[d1][d2] = 0;
int los_cnt = 0;
int win_cnt = 0;
forall(it,graph[v])
{
if(it != pop)
{
dfs1(it,v);
if(!dp_node[it].is_win) los_cnt++;
else win_cnt++;
dp_node[v].sub_siz += dp_node[it].sub_siz;
}
}
if(los_cnt > 0) dp_node[v].is_win = 1;
if(los_cnt >= 2)
{
dp_node[v].dp[1][0] = dp_node[v].sub_siz;
dp_node[v].dp[1][1] = dp_node[v].sub_siz;
dp_node[v].dp[0][0] = 0;
dp_node[v].dp[0][1] = 0;
}
else if(los_cnt == 1)
{
int los_vert;
forall(it,graph[v])
{
if(it != pop && !dp_node[it].is_win) los_vert = it;
}
dp_node[v].dp[1][1] = dp_node[v].sub_siz - dp_node[los_vert].sub_siz + dp_node[los_vert].dp[0][1];
dp_node[v].dp[1][0] = dp_node[v].sub_siz - dp_node[los_vert].sub_siz + dp_node[los_vert].dp[0][0];
dp_node[v].dp[0][1] = dp_node[los_vert].dp[1][1];
dp_node[v].dp[0][0] = dp_node[los_vert].dp[1][0];
}
else
{
dp_node[v].dp[1][0] = 1;
dp_node[v].dp[0][1] = 1;
forall(it,graph[v])
{
if(it != pop)
{
dp_node[v].dp[1][1] += dp_node[it].dp[0][1];
dp_node[v].dp[1][0] += dp_node[it].dp[0][0];
dp_node[v].dp[0][1] += dp_node[it].dp[1][1];
dp_node[v].dp[0][0] += dp_node[it].dp[1][0];
}
}
}
}
int main()
{
ios_base::sync_with_stdio(0);cin.tie(0);cout.tie(0);
//random_start();
int n;
ll D;
cin >> n >> D;
rep(i,n-1)
{
int a,b;
cin >> a >> b;
graph[a].pb(b);
graph[b].pb(a);
}
dfs1(1,1);
dp_ beg_tree;
dp_ mid_tree;
dp_ end_tree;
rep(d1,2) rep(d2,2)
{
beg_tree.dp[d1][d2] = dp_node[1].dp[d1][d2];
}
rep2(i,1,n)
{
dfs1(i,i);
if(dp_node[i].is_win) end_tree.dp[1][1]++;
else end_tree.dp[0][1]++;
rep(d1,2) rep(d2,2)
{
mid_tree.dp[d1][d2] += dp_node[i].dp[d1][d2];
mid_tree.dp[d1][d2] %= MOD;
}
}
D--;
rep2(bit,0,61)
{
if(D & (1LL << bit))
{
beg_tree = beg_tree + mid_tree;
}
mid_tree = mid_tree + mid_tree;
}
beg_tree = beg_tree + end_tree;
cout << beg_tree.dp[1][1] << "\n";
}
