import sys

input = sys.stdin.readline
sys.setrecursionlimit(2*10**5)

mod = 10**9+7

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

depth = [-1 for i in range(N)]
parent = [-1 for i in range(N)]
size = [1 for i in range(N)]
depth[0] = 0
def dfs(v,pv):
    for nv in edge[v]:
        if nv!=pv:
            parent[nv] = v
            depth[nv] = depth[v] + 1
            dfs(nv,v)
            size[v] += size[nv]

dfs(0,-1)

D = max(depth)

def solve(M):
    if M<=2*D+1:
        dp = [[1 for i in range(M)] for j in range(N)]
        def dp_dfs(v,pv):
            for nv in edge[v]:
                if nv!=pv:
                    dp_dfs(nv,v)
                    for i in range(M):
                        tmp = 0
                        if i!=0:
                            tmp += dp[nv][i-1]
                        if i!=M-1:
                            tmp += dp[nv][i+1]
                        tmp %= mod
                        dp[v][i] *= tmp
                        dp[v][i] %= mod

        dp_dfs(0,-1)
        res = 0
        for i in range(M):
            res += dp[0][i]
            res %= mod
        return res

    else:
        dp_low = [[1 for i in range(D+1)] for j in range(N)]
        dp_high = [[1 for i in range(D+1)] for j in range(N)]
        def dp_dfs(v,pv):
            for nv in edge[v]:
                if nv!=pv:
                    dp_dfs(nv,v)
                    for i in range(D+1):
                        tmp = 0
                        if i!=0:
                            tmp += dp_low[nv][i-1]
                        if i!=D:
                            tmp += dp_low[nv][i+1]
                        else:
                            tmp += pow(2,size[nv]-1,mod)
                        tmp %= mod
                        dp_low[v][i] *= tmp
                        dp_low[v][i] %= mod

                        tmp = 0
                        if i!=0:
                            tmp += dp_high[nv][i-1]
                        if i!=D:
                            tmp += dp_high[nv][i+1]
                        else:
                            tmp += pow(2,size[nv]-1,mod)
                        tmp %= mod
                        dp_high[v][i] *= tmp
                        dp_high[v][i] %= mod

        dp_dfs(0,-1)

        res = 0
        for i in range(D+1):
            res += dp_low[0][i]
            res %= mod
            res += dp_high[0][i]
            res %= mod

        tmp = pow(2,N-1,mod) * (M-2*D-2) %mod
        res += tmp
        res %= mod
        return res

ans = 0
c = C//3

if C%3==1:
    ans += solve(c+1) + 2*solve(c)
    ans %= mod
elif C%3==2:
    ans += 2*solve(c+1) + solve(c)
    ans %= mod
else:
    ans = 3*solve(c)
    ans %= mod

print(ans)