const MOD: i64 = 1_000_000_007; fn main() { let mut io = IO::new(); input!{ from io, n: usize, c: i64, ed: [(Usize1, Usize1); n-1] } let mut g = UndirectedGraph::new(n); for &(u, v) in &ed { g.add_edge(u, v, Void()); } let (_dist, par, _size, euler) = tree_dfs(&g, 0); let mut ans = 0; for i in 0..3 { ans = (ans + solve(&euler, &par, (c + 2 - i) / 3)) % MOD; } io.println(ans); } fn solve(euler: &[usize], par: &[Option], c: i64) -> i64 { let n = par.len(); let m = std::cmp::min(c as usize, n * 2 + 1); if m == 1 { return 0; } let mut dp = vec![vec![1; m]; n]; for &u in euler.iter().skip(1).rev() { let v = par[u].unwrap(); dp[v][0] = dp[v][0] * dp[u][1] % MOD; for i in 1..m-1 { dp[v][i] = dp[v][i] * (dp[u][i-1] + dp[u][i+1]) % MOD; } dp[v][m-1] = dp[v][m-1] * dp[u][m-2] % MOD; } let ans = if m == c as usize { dp[0].iter().sum::() } else { dp[0][..n].iter().sum::() * 2 + dp[0][n] * (c - 2 * n as i64) } % MOD; ans } pub fn tree_dfs>(g: &G, root: usize) -> (Vec, Vec>, Vec, Vec) { let n = g.size(); let mut euler = Vec::with_capacity(n); let mut dist = vec![C::MAX; n]; dist[root] = C::zero(); let mut par = vec![None; n]; let mut size = vec![1; n]; let mut q = vec![root]; while let Some(v) = q.pop() { euler.push(v); for e in g.edges_from(v) { if par[v] == Some(e.to) { continue; } par[e.to] = Some(v); dist[e.to] = dist[v] + e.cost; q.push(e.to); } } for &v in euler.iter().skip(1).rev() { size[par[v].unwrap()] += size[v]; } (dist, par, size, euler) } // ------------ Graph impl start ------------ pub trait Cost: Element + Clone + Copy + std::fmt::Display + Eq + Ord + Zero + One + Add + AddAssign + Sub + Neg { const MAX: Self; } #[derive(Copy, Clone)] pub struct Edge { // pub from: usize, pub to: usize, pub cost: C, pub id: usize } pub struct UndirectedGraph(pub Vec>>, pub usize); pub struct DirectedGraph{ pub forward: Vec>>, pub backward: Vec>>, pub count: usize, } pub trait Graph { fn new(size: usize) -> Self; fn size(&self) -> usize; fn add_edge(&mut self, u: usize, v: usize, cost: C); fn edges_from(&self, v: usize) -> std::slice::Iter>; } impl Graph for UndirectedGraph { fn new(size: usize) -> Self { Self(vec![Vec::>::new(); size], 0) } fn size(&self) -> usize { self.0.len() } fn add_edge(&mut self, u: usize, v: usize, cost: C) { self.0[u].push(Edge{ to: v, cost: cost.clone(), id: self.1 }); self.0[v].push(Edge{ to: u, cost: cost.clone(), id: self.1 }); self.1 += 1; } fn edges_from(&self, v: usize) -> std::slice::Iter> { self.0[v].iter() } } impl Graph for DirectedGraph { fn new(size: usize) -> Self { Self { forward: vec![Vec::>::new(); size], backward: vec![Vec::>::new(); size], count: 0 } } fn size(&self) -> usize { self.forward.len() } fn add_edge(&mut self, u: usize, v: usize, cost: C) { self.forward[u].push(Edge{ to: v, cost: cost.clone(), id: self.count }); self.backward[v].push(Edge{ to: u, cost: cost.clone(), id: self.count }); self.count += 1; } fn edges_from(&self, v: usize) -> std::slice::Iter> { self.forward[v].iter() } } impl DirectedGraph { pub fn edges_to(&self, u: usize) -> std::slice::Iter> { self.backward[u].iter() } pub fn reverse(&self) -> Self { Self { forward: self.backward.clone(), backward: self.forward.clone(), count: self.count, } } } macro_rules! impl_cost { ($($T:ident,)*) => { $( impl Cost for $T { const MAX: Self = std::$T::MAX; } )* }; } impl_cost! { i8, i16, i32, i64, i128, isize, } #[derive(Clone, Copy, PartialEq, Eq, PartialOrd, Ord)] pub struct Void(); impl std::fmt::Display for Void { fn fmt(&self, f: &mut std::fmt::Formatter) -> std::fmt::Result { write!(f, "") } } impl Zero for Void { fn zero() -> Self { Void() } fn is_zero(&self) -> bool { true } } impl One for Void { fn one() -> Self { Void() } fn is_one(&self) -> bool { true } } impl Add for Void { type Output = Self; fn add(self, _: Self) -> Self { Void() } } impl AddAssign for Void { fn add_assign(&mut self, _: Self) {} } impl Sub for Void { type Output = Self; fn sub(self, _: Self) -> Self { Void() } } impl Neg for Void { type Output = Self; fn neg(self) -> Self { Void() } } impl Cost for Void { const MAX: Self = Void(); } // ------------ Graph impl end ------------ // ------------ algebraic traits start ------------ use std::marker::Sized; use std::ops::*; /// 元 pub trait Element: Sized + Clone + PartialEq {} impl Element for T {} /// 結合性 pub trait Associative: Magma {} /// マグマ pub trait Magma: Element + Add {} impl> Magma for T {} /// 半群 pub trait SemiGroup: Magma + Associative {} impl SemiGroup for T {} /// モノイド pub trait Monoid: SemiGroup + Zero {} impl Monoid for T {} pub trait ComMonoid: Monoid + AddAssign {} impl ComMonoid for T {} /// 群 pub trait Group: Monoid + Neg {} impl> Group for T {} pub trait ComGroup: Group + ComMonoid {} impl ComGroup for T {} /// 半環 pub trait SemiRing: ComMonoid + Mul + One {} impl + One> SemiRing for T {} /// 環 pub trait Ring: ComGroup + SemiRing {} impl Ring for T {} pub trait ComRing: Ring + MulAssign {} impl ComRing for T {} /// 体 pub trait Field: ComRing + Div + DivAssign {} impl + DivAssign> Field for T {} /// 加法単元 pub trait Zero: Element { fn zero() -> Self; fn is_zero(&self) -> bool { *self == Self::zero() } } /// 乗法単元 pub trait One: Element { fn one() -> Self; fn is_one(&self) -> bool { *self == Self::one() } } macro_rules! impl_integer { ($($T:ty,)*) => { $( impl Associative for $T {} impl Zero for $T { fn zero() -> Self { 0 } fn is_zero(&self) -> bool { *self == 0 } } impl<'a> Zero for &'a $T { fn zero() -> Self { &0 } fn is_zero(&self) -> bool { *self == &0 } } impl One for $T { fn one() -> Self { 1 } fn is_one(&self) -> bool { *self == 1 } } impl<'a> One for &'a $T { fn one() -> Self { &1 } fn is_one(&self) -> bool { *self == &1 } } )* }; } impl_integer! { i8, i16, i32, i64, i128, isize, u8, u16, u32, u64, u128, usize, } // ------------ algebraic traits end ------------ // ------------ io module start ------------ use std::io::{stdout, BufWriter, Read, StdoutLock, Write}; pub struct IO { iter: std::str::SplitAsciiWhitespace<'static>, buf: BufWriter>, } impl IO { pub fn new() -> Self { let mut input = String::new(); std::io::stdin().read_to_string(&mut input).unwrap(); let input = Box::leak(input.into_boxed_str()); let out = Box::new(stdout()); IO { iter: input.split_ascii_whitespace(), buf: BufWriter::new(Box::leak(out).lock()), } } fn scan_str(&mut self) -> &'static str { self.iter.next().unwrap() } pub fn scan(&mut self) -> ::Output { ::scan(self) } pub fn scan_vec(&mut self, n: usize) -> Vec<::Output> { (0..n).map(|_| self.scan::()).collect() } pub fn print(&mut self, x: T) { ::print(self, x); } pub fn println(&mut self, x: T) { self.print(x); self.print("\n"); } pub fn iterln>(&mut self, mut iter: I, delim: &str) { if let Some(v) = iter.next() { self.print(v); for v in iter { self.print(delim); self.print(v); } } self.print("\n"); } pub fn flush(&mut self) { self.buf.flush().unwrap(); } } impl Default for IO { fn default() -> Self { Self::new() } } pub trait Scan { type Output; fn scan(io: &mut IO) -> Self::Output; } macro_rules! impl_scan { ($($t:tt),*) => { $( impl Scan for $t { type Output = Self; fn scan(s: &mut IO) -> Self::Output { s.scan_str().parse().unwrap() } } )* }; } impl_scan!(i16, i32, i64, isize, u16, u32, u64, usize, String); pub enum Bytes {} impl Scan for Bytes { type Output = &'static [u8]; fn scan(s: &mut IO) -> Self::Output { s.scan_str().as_bytes() } } pub enum Chars {} impl Scan for Chars { type Output = Vec; fn scan(s: &mut IO) -> Self::Output { s.scan_str().chars().collect() } } pub enum Usize1 {} impl Scan for Usize1 { type Output = usize; fn scan(s: &mut IO) -> Self::Output { s.scan::().wrapping_sub(1) } } impl Scan for (T, U) { type Output = (T::Output, U::Output); fn scan(s: &mut IO) -> Self::Output { (T::scan(s), U::scan(s)) } } impl Scan for (T, U, V) { type Output = (T::Output, U::Output, V::Output); fn scan(s: &mut IO) -> Self::Output { (T::scan(s), U::scan(s), V::scan(s)) } } impl Scan for (T, U, V, W) { type Output = (T::Output, U::Output, V::Output, W::Output); fn scan(s: &mut IO) -> Self::Output { (T::scan(s), U::scan(s), V::scan(s), W::scan(s)) } } pub trait Print { fn print(w: &mut IO, x: Self); } macro_rules! impl_print_int { ($($t:ty),*) => { $( impl Print for $t { fn print(w: &mut IO, x: Self) { w.buf.write_all(x.to_string().as_bytes()).unwrap(); } } )* }; } impl_print_int!(i16, i32, i64, isize, u16, u32, u64, usize); impl Print for u8 { fn print(w: &mut IO, x: Self) { w.buf.write_all(&[x]).unwrap(); } } impl Print for &[u8] { fn print(w: &mut IO, x: Self) { w.buf.write_all(x).unwrap(); } } impl Print for &str { fn print(w: &mut IO, x: Self) { w.print(x.as_bytes()); } } impl Print for String { fn print(w: &mut IO, x: Self) { w.print(x.as_bytes()); } } impl Print for (T, U) { fn print(w: &mut IO, (x, y): Self) { w.print(x); w.print(" "); w.print(y); } } impl Print for (T, U, V) { fn print(w: &mut IO, (x, y, z): Self) { w.print(x); w.print(" "); w.print(y); w.print(" "); w.print(z); } } mod neboccoio_macro { #[macro_export] macro_rules! input { (@start $io:tt @read @rest) => {}; (@start $io:tt @read @rest, $($rest: tt)*) => { input!(@start $io @read @rest $($rest)*) }; (@start $io:tt @read @rest mut $($rest:tt)*) => { input!(@start $io @read @mut [mut] @rest $($rest)*) }; (@start $io:tt @read @rest $($rest:tt)*) => { input!(@start $io @read @mut [] @rest $($rest)*) }; (@start $io:tt @read @mut [$($mut:tt)?] @rest $var:tt: [$kind:tt; $len:expr] $($rest:tt)*) => { let $($mut)* $var = $io.scan_vec::<$kind>($len); input!(@start $io @read @rest $($rest)*) }; (@start $io:tt @read @mut [$($mut:tt)?] @rest $var:tt: $kind:tt $($rest:tt)*) => { let $($mut)* $var = $io.scan::<$kind>(); input!(@start $io @read @rest $($rest)*) }; (from $io:tt $($rest:tt)*) => { input!(@start $io @read @rest $($rest)*) }; } } // ------------ io module end ------------