#include <bits/stdc++.h>
#define int long long
using namespace std;
const int MAX_N = 1e5;
const int MAX_D = 1e5;
const int MOD = 1e9 + 7;
bool canWin[MAX_N + 1];
int countChildrenLosers[MAX_N + 1], reach[MAX_N + 1], critical[MAX_N + 1], dp[MAX_D];
vector<int> adj[MAX_N + 1];
void dfsGame( int u, int p ) {
canWin[u] = false;
countChildrenLosers[u] = 0;
for ( int v: adj[u] ) {
if ( v == p )
continue;
dfsGame( v, u );
canWin[u] |= (!canWin[v]);
countChildrenLosers[u] += (!canWin[v]);
}
}
bool influence( int u, int p ) {
if ( p == 0 )
return false;
if ( canWin[u] ) {
if ( canWin[p] )
return false;
return true;
}
if ( countChildrenLosers[p] >= 2 )
return false;
return true;
}
void dfsCritical( int u, int p ) {
critical[u] = (!canWin[u]);
for ( int v: adj[u] ) {
if ( v == p )
continue;
dfsCritical( v, u );
if ( influence( v, u ) )
critical[u] += critical[v];
}
}
int losers = 0, c = 0;
void reroot( int u, int p ) {
losers += !canWin[u];
c += (canWin[u] ? 1 : -1) * critical[u];
/*for ( int v = 1; v <= 4; v++ )
printf( "%d ", critical[v] );
printf( "\n" );*/
for ( int v: adj[u] ) {
if ( v == p )
continue;
if ( influence( v, u ) )
critical[u] -= critical[v];
//printf( "ch %d %d\n", critical[u], critical[v] );
critical[v] -= (!canWin[v]);
critical[u] -= (!canWin[u]);
countChildrenLosers[u] -= (!canWin[v]);
canWin[u] = (countChildrenLosers[u] > 0);
countChildrenLosers[v] += (!canWin[u]);
canWin[v] = (countChildrenLosers[v] > 0);
critical[v] += (!canWin[v]);
critical[u] += (!canWin[u]);
if ( influence( u, v ) )
critical[v] += critical[u];
reroot( v, u );
if ( influence( u, v ) )
critical[v] -= critical[u];
critical[v] -= (!canWin[v]);
critical[u] -= (!canWin[u]);
countChildrenLosers[v] -= (!canWin[u]);
canWin[v] = (countChildrenLosers[v] > 0);
countChildrenLosers[u] += (!canWin[v]);
canWin[u] = (countChildrenLosers[u] > 0);
critical[v] += (!canWin[v]);
critical[u] += (!canWin[u]);
if ( influence( v, u ) )
critical[u] += critical[v];
}
}
int lgPut( int x, int n ) {
if ( n == 0 )
return 1;
int p = lgPut( x, n / 2 );
p = (long long)p * p % MOD;
if ( n % 2 == 1 )
p = (long long)p * x % MOD;
return p;
}
signed main() {
int n, d;
cin >> n >> d;
for ( int i = 0; i < n - 1; i++ ) {
int u, v;
cin >> u >> v;
adj[u].push_back( v );
adj[v].push_back( u );
}
dfsGame( 1, 0 );
dfsCritical( 1, 0 );
reroot( 1, 0 );
// cout << "C: " << c << "\n";
c = 0;
for ( int r = 1; r <= n; r++ ) {
dfsGame( r, 0 );
dfsCritical( r, 0 );
//cout << r << " " << critical[r] << "\n";
c += (canWin[r] ? 1 : -1) * critical[r];
}
//cout << "C: " << c << "\n";
while ( c < 0 )
c += MOD;
dp[0] = losers;
for ( int i = 1; i < d; i++ )
dp[i] = (losers * lgPut( n, 2 * i ) + c * dp[i - 1]) % MOD;
dfsGame( 1, 0 );
dfsCritical( 1, 0 );
if ( canWin[1] )
cout << (lgPut( n, 2 * d ) - dp[d - 1] * critical[1] % MOD + MOD) % MOD;
else
cout << dp[d - 1] * critical[1] % MOD << "\n";
return 0;
}
