#pragma once
#include <bits/stdc++.h>
using namespace std;
#define fileio() freopen("input.txt", "r", stdin), freopen("output.txt", "w", stdout)
#define fastio() cin.tie(0), ios_base::sync_with_stdio(0)
#define sp " "
#define endl "\n"
#define pb push_back
#define pii pair<int, int>
#define st first
#define nd second
#define N 100005
#define ll long long
const int modulo = 1e9 + 7;
ll mul(ll a, ll b)
{
return (a * b) % modulo;
}
ll add(ll a, ll b)
{
if (a + b < modulo) return a + b;
return a + b - modulo;
}
ll subs(ll a, ll b)
{
if (a < b) return a - b + modulo;
return a - b;
}
int dp[N][2][2], reach[2][N], edge[N][2];
vector<pii> adj[N];
inline int f(int node, int root, int turn);
inline void dfs(int ind, int j, int dep, int turn)
{
int node = edge[ind][j];
int root = edge[ind][1 - j];
int player = dep % 2;
if (player == turn) reach[turn][node] = 1;
int cnt = 0;
for (auto i : adj[node])
{
int curr = edge[i.st][i.nd];
if (curr == root) continue;
if (f(i.st, i.nd, 1 - player) == 1 - turn) cnt++;
}
if (player == turn || cnt == 1)
{
for (auto i : adj[node])
{
int curr = edge[i.st][i.nd];
if (curr == root) continue;
if (turn != player && f(i.st, i.nd, 1 - player) != player) continue;
dfs(i.st, i.nd, dep + 1, turn);
}
}
}
inline int gh(int node, int turn)
{
int ans = 0;
if (turn == 0) ans = 1;
for (auto i : adj[node])
{
if (turn == 0) ans &= f(i.st, i.nd, 1 - turn);
else ans |= f(i.st, i.nd, 1 - turn);
}
return ans;
}
inline int f(int ind, int j, int turn)
{
if(dp[ind][j][turn] != -1) return dp[ind][j][turn];
int ans = 0;
if (turn == 0) ans = 1;
for (auto i : adj[edge[ind][j]])
{
if (i.st == ind) continue;
if (turn == 0) ans &= f(i.st, i.nd, 1 - turn);
else ans |= f(i.st, i.nd, 1 - turn);
}
return dp[ind][j][turn] = ans;
}
int32_t main()
{
#ifndef ONLINE_JUDGE
//fileio();
#endif
fastio();
memset(dp, -1, sizeof(dp));
int n, d;
cin>>n>>d;
edge[0][0] = 1;
for (int i = 1; i < n; i++)
{
int u, v;
cin>>u>>v;
adj[u].pb({i, 0});
adj[v].pb({i, 1});
edge[i][0] = v;
edge[i][1] = u;
}
int res = gh(1, 1);
int sum = 0;
int agn = 0, gab = 0;
for (int i = 1; i <= n; i++) if (gh(i, 0)) sum++;
int ans = 0;
if (res == 1)
{
dfs(0, 0, 1, 0);
for (int i = 1; i <= n; i++) if (reach[0][i]) gab++;
ans = mul(n, n);
ans = subs(ans, mul(sum, gab));
}
else
{
dfs(0, 0, 1, 1);
for (int i = 1; i <= n; i++) if (reach[1][i]) agn++;
ans = mul(agn, sum);
}
cout<<ans<<endl;
cerr<<"time taken : "<<clock() / CLOCKS_PER_SEC<<" seconds\n";
}
