#include #define rep(i,n) for (int i = 0; i < (n); ++i) #define chmax(a, b) a = max(a, b) #define chmin(a, b) a = min(a, b) #define all(x) (x).begin(), (x).end() using namespace std; using ll = long long; using P = pair; using VI = vector; using VVI = vector; #define MOD 1000000007 //#define MOD 998244353 struct mint { int i; mint() : i(0) {} mint(int x) { i = int(x % MOD); if (i < 0) i += MOD; } template mint(T x) { i = int(x % MOD); if (i < 0) i += MOD; } mint operator+(const mint x) const {return i + x.i;} mint operator-(const mint x) const {return i - x.i;} mint operator*(const mint x) const {return (long long)i * x.i;} mint operator/(const mint x) const {return (long long)i * x.pow(MOD - 2).i;} mint inv() {return pow(MOD - 2);} template mint pow(T p) const { long long r = 1; long long t = i; for(; p > 0; p >>= 1) { if (p & 1) r = r * t % MOD; t = t * t % MOD; } return r; } template static mint pow(T1 a, T2 b) { long long r = 1; long long t = (long long)(a % MOD); for(; b > 0; b >>= 1) { if (b & 1) r = r * t % MOD; t = t * t % MOD; } return r; } mint& operator+=(const mint x) { i = (i + x.i) % MOD; return *this; } mint& operator-=(const mint x) { i = i - x.i; if (i < 0) i += MOD; return *this; } mint& operator*=(const mint x) { i = (int)((long long)i * x.i % MOD); return *this; } mint& operator/=(const mint x) { i = (long long)i * x.pow(MOD - 2).i % MOD; return *this; } }; std::ostream& operator<<(std::ostream& os, const mint& m) { return os << m.i; } VVI to; int n, c; vector init_v; bool first_init = true; int sz; vector dfs(int u=0, int p=-1) { vector dp(sz); vector> cdp; for(int v: to[u]) { if (v == p) continue; cdp.push_back(dfs(v, u)); } if (cdp.size() == 0) { first_init = false; return init_v; } rep(i, sz) { mint t = 1; for(auto& d: cdp) { mint acc = 0; if (i - 1 >= 0) acc += d[i-1]; if (i + 1 < sz) acc += d[i+1]; t *= acc; } dp[i] = t; } if (p == -1) first_init = true; return dp; } int main() { cin >> n >> c; to.resize(n); rep(i, n-1) { int a, b; cin >> a >> b; a--; b--; to[a].push_back(b); to[b].push_back(a); } if (c <= 6100) { auto f = [&]() { init_v = vector(sz); rep(i, sz) init_v[i] = 1; auto a = dfs(); mint ret = 0; rep(i, sz) ret += a[i]; return ret; }; int c1 = c / 3; int r = c % 3; sz = c1; mint ans = f() * (3 - r); sz++; ans += f() * r; cout << ans << endl; } else { sz = 2000; init_v = vector(sz); rep(i, 999) init_v[i] = 1; auto v_bounded = dfs(); mint s_bounded = 0; rep(i, sz) s_bounded += v_bounded[i]; init_v = vector(sz); init_v[999] = 1; auto v_unbounded = dfs(); mint s_unbounded = 0; rep(i, sz) s_unbounded += v_unbounded[i]; mint ans = s_bounded * 6; int c1 = c / 3; int r = c % 3; ans += s_unbounded * (c1 - 999 * 2) * (3 - r); ans += s_unbounded * (c1 + 1 - 999 * 2) * r; cout << ans << endl; } }