#include using namespace std; #define fs first #define sc second #define pb push_back #define mp make_pair #define eb emplace_back #define ALL(A) A.begin(),A.end() #define RALL(A) A.rbegin(),A.rend() typedef long long LL; typedef pair P; template inline bool chmax(T& a, T b) { if (a < b) { a = b; return 1; } return 0; } template inline bool chmin(T& a, T b) { if (a > b) { a = b; return 1; } return 0; } template T gcd(T a,T b){return b?gcd(b,a%b):a;} const LL mod=1000000007; const LL LINF=1LL<<60; const int INF=1<<30; int dx[]={1,0,-1,0,1,-1,1,-1}; int dy[]={0,1,0,-1,1,-1,-1,1}; int n,c; vector G[1<<10]; vector> dp(1<<10, vector (10000, -1)); LL dfs(int u,int v,int s){ if(~dp[u][s]) return dp[u][s]; LL ret = 1; for(auto g:G[u]){ if(g == v) continue; LL tmp = 0; if(s + 3 < c) tmp = (tmp + dfs(g, u, s + 3))%mod; if(s - 3 >= 0) tmp = (tmp + dfs(g, u, s - 3))%mod; ret = (ret * tmp)%mod; } return dp[u][s] = ret; } LL fpow(LL a, LL n){ LL x = 1; while(n > 0){ if(n&1){ x=x*a%mod; } a=a*a%mod; n >>= 1; } return x; } int main(){ cin >> n >> c; for (int i = 0; i < n-1; i++) { int a,b;cin >> a >> b; a--,b--; G[a].pb(b); G[b].pb(a); } LL ans = 0; if(c <= 8000){ for (int i = 0; i < c; i++) { ans = (ans + dfs(0, -1, i))%mod; } } else{ for (int i = 0; i <= 4000; i++) { ans = (ans + 2 * dfs(0, -1, i))%mod; } ans = (ans + (c - 8000) * dp[0][4000])%mod; } cout << ans << endl; }