#i hate three
from collections import deque

mod = 10**9 + 7


def main():
    N, C = map(int, input().split())
    G = [[] for i in range(N+1)]
    for i in range(N-1):
        a, b = map(int, input().split())
        G[a].append(b)
        G[b].append(a)

    visited = [False for i in range(N+1)]
    depth = [0 for i in range(N+1)]
    child_cnt = [1 for i in range(N+1)]

    que = deque([])

    def dfs(s_node):
        visited[s_node] = True
        for child in G[s_node]:
            if visited[child] == False:
                depth[child] = depth[s_node] + 1
                dfs(child)
        for child in G[s_node]:
            if depth[child] > depth[s_node]:
                child_cnt[s_node] += child_cnt[child]
        que.append(s_node)

    dfs(1)

    if C > 9*N:
        dp = [[0 for i in range(6*N-5)] for i in range(N+1)]
        while que:
            node = que.popleft()
            for i in range(3*N-4):
                has_child = False
                v = 1
                for child in G[node]:
                    if depth[child] > depth[node]:
                        tmp_u = 0
                        has_child = True
                        if i >= 3:
                            tmp_u += dp[child][i-3]

                        if i+3 <= 3*N-5:
                            tmp_u += dp[child][i+3]

                        elif i+3 > 3*N-5:
                            tmp_u += pow(2, child_cnt[child]-1, mod)

                        v *= tmp_u
                        v %= mod
                dp[node][i] = v
                if has_child == False:
                    dp[node][i] = 1

            for i in range(3*N-3, 6*N-5):
                has_child = False
                v = 1
                for child in G[node]:
                    if depth[child] > depth[node]:

                        tmp_u = 0
                        has_child = True
                        if i+3 <= 6*N-6:
                            tmp_u += dp[child][i+3]
                        if i-3 >= 3*N-3:
                            tmp_u += dp[child][i-3]
                        else:
                            tmp_u += pow(2, child_cnt[child]-1, mod)

                        v *= tmp_u
                        v %= mod
                dp[node][i] = v
                if has_child == False:
                    dp[node][i] = 1

        ans = sum(dp[1][i] for i in range(3*N-4)) + sum(dp[1][i]
                                                        for i in range(3*N-3, 6*N-5)) + (C-6*N + 6)*pow(2, N-1, mod)
        print(ans % mod)
        return

    elif C <= 9*N:
        dp = [[0 for i in range(C+1)] for j in range(N+1)]
        while que:
            node = que.popleft()
            for i in range(1, C+1):
                has_child = False
                v = 1
                for child in G[node]:
                    if depth[child] > depth[node]:
                        has_child = True
                        tmp_u = 0
                        if 1 <= i-3 <= C:
                            tmp_u += dp[child][i-3]
                        if 1 <= i+3 <= C:
                            tmp_u += dp[child][i+3]
                        v *= (tmp_u)
                        v %= mod
                dp[node][i] = v % mod
                if has_child == False:
                    dp[node][i] = 1

        ans = sum(dp[1][i] for i in range(1, C+1)) % mod
        print(ans)
        return


if __name__ == "__main__":
    main()