#include <iostream>
#include <vector>

template <int MOD>
struct ModInt {
    using lint = long long;
    int val;

    // constructor
    ModInt(lint v = 0) : val(v % MOD) {
        if (val < 0) val += MOD;
    };

    // unary operator
    ModInt operator+() const { return ModInt(val); }
    ModInt operator-() const { return ModInt(MOD - val); }
    ModInt inv() const { return this->pow(MOD - 2); }

    // arithmetic
    ModInt operator+(const ModInt& x) const { return ModInt(*this) += x; }
    ModInt operator-(const ModInt& x) const { return ModInt(*this) -= x; }
    ModInt operator*(const ModInt& x) const { return ModInt(*this) *= x; }
    ModInt operator/(const ModInt& x) const { return ModInt(*this) /= x; }
    ModInt pow(lint n) const {
        auto x = ModInt(1);
        auto b = *this;
        while (n > 0) {
            if (n & 1) x *= b;
            n >>= 1;
            b *= b;
        }
        return x;
    }

    // compound assignment
    ModInt& operator+=(const ModInt& x) {
        if ((val += x.val) >= MOD) val -= MOD;
        return *this;
    }
    ModInt& operator-=(const ModInt& x) {
        if ((val -= x.val) < 0) val += MOD;
        return *this;
    }
    ModInt& operator*=(const ModInt& x) {
        val = lint(val) * x.val % MOD;
        return *this;
    }
    ModInt& operator/=(const ModInt& x) { return *this *= x.inv(); }

    // compare
    bool operator==(const ModInt& b) const { return val == b.val; }
    bool operator!=(const ModInt& b) const { return val != b.val; }
    bool operator<(const ModInt& b) const { return val < b.val; }
    bool operator<=(const ModInt& b) const { return val <= b.val; }
    bool operator>(const ModInt& b) const { return val > b.val; }
    bool operator>=(const ModInt& b) const { return val >= b.val; }

    // I/O
    friend std::istream& operator>>(std::istream& is, ModInt& x) noexcept {
        lint v;
        is >> v;
        x = v;
        return is;
    }
    friend std::ostream& operator<<(std::ostream& os, const ModInt& x) noexcept { return os << x.val; }
};

template <class Cost = int>
struct Edge {
    int src, dst;
    Cost cost;
    Edge(int src = -1, int dst = -1, Cost cost = 1)
        : src(src), dst(dst), cost(cost){};

    bool operator<(const Edge<Cost>& e) const { return this->cost < e.cost; }
    bool operator>(const Edge<Cost>& e) const { return this->cost > e.cost; }
};

template <class Cost = int>
struct Graph {
    std::vector<std::vector<Edge<Cost>>> graph;

    Graph(int n = 0) : graph(n) {}

    void span(bool direct, int src, int dst, Cost cost = 1) {
        graph[src].emplace_back(src, dst, cost);
        if (!direct) graph[dst].emplace_back(dst, src, cost);
    }

    int size() const { return graph.size(); }
    void clear() { graph.clear(); }
    void resize(int n) { graph.resize(n); }

    std::vector<Edge<Cost>>& operator[](int v) { return graph[v]; }
    std::vector<Edge<Cost>> operator[](int v) const { return graph[v]; }
};

constexpr int MOD = 1000000007;
using mint = ModInt<MOD>;

Graph<> graph;
int sz;

std::vector<mint> dfs(int v, int p) {
    std::vector<mint> dp(sz, 1);

    for (auto e : graph[v]) {
        int u = e.dst;
        if (u == p) continue;

        auto cdp = dfs(u, v);
        for (int i = 0; i < sz; ++i) {
            mint sum = 0;
            for (int j = -1; j <= 1; j += 2) {
                int ni = i + j;
                if (ni < 0 || sz <= ni) continue;
                sum += cdp[ni];
            }
            dp[i] *= sum;
        }
    }

    return dp;
}

mint calc(int k) {
    if (k < 5000) {
        sz = k;
        auto dp = dfs(0, -1);

        mint ret = 0;
        for (int i = 0; i < k; ++i) ret += dp[i];
        return ret;
    }

    sz = 3000;
    auto dp = dfs(0, -1);

    mint ret = 0;
    for (int i = 0; i < 1000; ++i) ret += dp[i];

    ret *= 2;
    ret += mint(2).pow(graph.size() - 1) * (k - 2000);

    return ret;
}

void solve() {
    int n, c;
    std::cin >> n >> c;

    graph = Graph<>(n);
    for (int i = 0; i < n - 1; ++i) {
        int u, v;
        std::cin >> u >> v;
        graph.span(false, --u, --v);
    }

    mint ans = 0;
    for (int j = 0; j < 3; ++j) {
        ans += calc((c + j) / 3);
    }
    std::cout << ans << "\n";
}

int main() {
    std::cin.tie(nullptr);
    std::ios::sync_with_stdio(false);

    solve();

    return 0;
}