Buggy python submission (verdict: Time Limit Exceeded):
# 75

import sys
sys.setrecursionlimit(10**8)

MOD = 10**9+7
N = int(input())
g_l = [[] for i in range(N)]
check_l = [-1] * N
num_l = [0] * N
for i in range(N-1):
    ai, bi = map(int, input().split())
    g_l[ai-1].append(bi-1)
    g_l[bi-1].append(ai-1)


def dfs(n):
    d = 1
    if check_l[n] > 0:
        return num_l[n]
    check_l[n] = 1
    for next_n in g_l[n]:
        d += dfs(next_n)
    num_l[n] = d
    return d


def modpow(a, n):
    # calc : (a**n)%MOD
    ret = 1
    while n > 0:
        if n & 1:
            ret = (ret * a) % MOD
        a = (a * a) % MOD
        n >>= 1
    return ret


def modinv(a):
    # calc : (a**(-1)) %MOD
    return modpow(a, MOD-2)


dfs(0)

ans = 0
# for i in range(1, N):
#     ans += (1 - (1/2)**num_l[i]) * (1 - (1/2)**(N-num_l[i]))
# ans += 1 - (1/2)**N  # 辺の期待値→頂点の期待値に変換(空集合以外は+1される)
# ans -= N/2  # 黒点の期待値を引く
for i in range(1, N):
    ans = (ans + (pow(2, num_l[i], MOD)-1) *
           (pow(2, N-num_l[i])-1) % MOD) % MOD
ans = (ans + (pow(2, N, MOD) - 1) - (N * pow(2, N-1, MOD)) % MOD) % MOD
y = ans
x = pow(2, N, MOD)

z = (y * modinv(x)) % MOD

print(z)


Fixed submission:
# 75

import sys
sys.setrecursionlimit(10**8)

MOD = 10**9+7
N = int(input())
g_l = [[] for i in range(N)]
check_l = [-1] * N
num_l = [0] * N
for i in range(N-1):
    ai, bi = map(int, input().split())
    g_l[ai-1].append(bi-1)
    g_l[bi-1].append(ai-1)


def dfs(n):
    d = 1
    if check_l[n] > 0:
        return num_l[n]
    check_l[n] = 1
    for next_n in g_l[n]:
        d += dfs(next_n)
    num_l[n] = d
    return d


def modinv(a):
    # calc : (a**(-1)) %MOD
    return pow(a, MOD-2, MOD)


def solve():
    dfs(0)
    pow_l = [None]*(N+1)
    pow_l[0] = 1
    for i in range(1, N+1):
        pow_l[i] = (pow_l[i-1] * 2) % MOD

    ans = 0
    # for i in range(1, N):
    #     ans += (1 - (1/2)**num_l[i]) * (1 - (1/2)**(N-num_l[i]))
    # ans += 1 - (1/2)**N  # 辺の期待値→頂点の期待値に変換(空集合以外は+1される)
    # ans -= N/2  # 黒点の期待値を引く
    for i in range(1, N):
        ans = (ans + (pow_l[num_l[i]]-1) *
               (pow_l[N-num_l[i]]-1) % MOD) % MOD
    ans = (ans + (pow_l[N] - 1) - (N * pow_l[N-1]) % MOD) % MOD
    y = ans
    x = pow(2, N, MOD)

    z = (y * modinv(x)) % MOD

    print(z)


solve()

Buggy python submission (verdict: Runtime Error):
# 75

from fractions import Fraction

MOD = 10**9+7
N = int(input())
g_l = [[] for i in range(N)]
check_l = [-1] * N
num_l = [0] * N
for i in range(N-1):
    ai, bi = map(int, input().split())
    g_l[ai-1].append(bi-1)
    g_l[bi-1].append(ai-1)


def dfs(n):
    d = 1
    if check_l[n] > 0:
        return num_l[n]
    check_l[n] = 1
    for next_n in g_l[n]:
        d += dfs(next_n)
    num_l[n] = d
    return d


def modpow(a, n):
    # calc : (a**n)%MOD
    ret = 1
    while n > 0:
        if n & 1:
            ret = (ret * a) % MOD
        a = (a * a) % MOD
        n >>= 1
    return ret


def modinv(a):
    # calc : (a**(-1)) %MOD
    return modpow(a, MOD-2)


dfs(0)

ans = 0
for i in range(1, N):
    ans += (1 - (1/2)**num_l[i]) * (1 - (1/2)**(N-num_l[i]))
ans += 1 - (1/2)**N  # 辺の期待値→頂点の期待値に変換(空集合以外は+1される)
ans -= N/2  # 黒点の期待値を引く

tmp = Fraction(ans)
x = tmp.denominator
y = tmp.numerator

z = (modinv(x) * y) % MOD

print(z)


Fixed submission:
# 75

import sys
sys.setrecursionlimit(10**8)

MOD = 10**9+7
N = int(input())
g_l = [[] for i in range(N)]
check_l = [-1] * N
num_l = [0] * N
for i in range(N-1):
    ai, bi = map(int, input().split())
    g_l[ai-1].append(bi-1)
    g_l[bi-1].append(ai-1)


def dfs(n):
    d = 1
    if check_l[n] > 0:
        return num_l[n]
    check_l[n] = 1
    for next_n in g_l[n]:
        d += dfs(next_n)
    num_l[n] = d
    return d


def modinv(a):
    # calc : (a**(-1)) %MOD
    return pow(a, MOD-2, MOD)


def solve():
    dfs(0)
    pow_l = [None]*(N+1)
    pow_l[0] = 1
    for i in range(1, N+1):
        pow_l[i] = (pow_l[i-1] * 2) % MOD

    ans = 0
    # for i in range(1, N):
    #     ans += (1 - (1/2)**num_l[i]) * (1 - (1/2)**(N-num_l[i]))
    # ans += 1 - (1/2)**N  # 辺の期待値→頂点の期待値に変換(空集合以外は+1される)
    # ans -= N/2  # 黒点の期待値を引く
    for i in range(1, N):
        ans = (ans + (pow_l[num_l[i]]-1) *
               (pow_l[N-num_l[i]]-1) % MOD) % MOD
    ans = (ans + (pow_l[N] - 1) - (N * pow_l[N-1]) % MOD) % MOD
    y = ans
    x = pow(2, N, MOD)

    z = (y * modinv(x)) % MOD

    print(z)


solve()