Buggy python submission (verdict: Time Limit Exceeded):
import sys
sys.setrecursionlimit(10**6)
mod = 10**9+7
p2 = [1]*(3*10**5)
for n in range(3*10**5-1):
    p2[n+1] = p2[n]*2%mod

N = int(input())
AB = [list(map(int,input().split())) for _ in range(N-1)]
g = [[] for _ in range(N+1)]
for i in range(N-1):
    a,b = AB[i]
    g[a].append(b)
    g[b].append(a)

l1 = [0]*(N+1)
l2 = [[] for _ in range(N+1)]

def rec(p,c):
    res = 0
    for a in g[c]:
        if a == p:
            continue
        n = rec(c,a)
        l2[c].append(n+1)
        res += n+1
    l1[c] = res
    return res

n = rec(0,1)
for i in range(1,N+1):
    l2[i].append(N-1-l1[i])

ans = 0
for i in range(1,N+1):
    ans += p2[N-1]-1
    ans %= mod
    for j in l2[i]:
        ans -= p2[j]-1
        ans %= mod
ans *= pow(p2[N],mod-2,mod)
ans %= mod
print(ans)


Fixed submission:
from collections import defaultdict,deque
mod = 10**9+7
N = int(input())
g = [[] for _ in range(N)]
d = defaultdict(lambda:-1)
p2 = [1]*(N+1)
p2[1] = pow(2,mod-2,mod)
for i in range(N):
    p2[i+1] = (p2[i]*p2[1])%mod

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

q = deque([[a,b]])
while q:
    i,j = q.pop()
    s = 1
    f = True
    for k in g[j]:
        if k == i:
            continue
        if (j,k) not in d:
            if f:
                q.append([i,j])
            q.append([j,k])
            f = False
        else:
            s += d[(j,k)]
    if f:
        d[(i,j)] = s

q = deque([[b,a]])
while q:
    i,j = q.pop()
    s = 1
    f = True
    for k in g[j]:
        if k == i:
            continue
        if (j,k) not in d:
            if f:
                q.append([i,j])
            q.append([j,k])
            f = False
        else:
            s += d[(j,k)]
    if f:
        d[(i,j)] = s

ans = 1-p2[N]-N*p2[1]

for i in range(N):
    for j in g[i]:
        if j<i:
            continue
        n = max(d[(i,j)],d[(j,i)])
        ans += (1-p2[n])*(1-p2[N-n])
        ans %= mod

print(ans)

Buggy python submission (verdict: Runtime Error):
mod = 10**9+7
p2 = [1]*(3*10**5)
for n in range(3*10**5-1):
    p2[n+1] = p2[n]*2%mod

N = int(input())
AB = [list(map(int,input().split())) for _ in range(N-1)]
g = [[] for _ in range(N+1)]
for i in range(N-1):
    a,b = AB[i]
    g[a].append(b)
    g[b].append(a)

l1 = [0]*(N+1)
l2 = [[] for _ in range(N+1)]

def rec(p,c):
    res = 0
    for a in g[c]:
        if a == p:
            continue
        n = rec(c,a)
        l2[c].append(n+1)
        res += n+1
    l1[c] = res
    return res

n = rec(0,1)
for i in range(1,N+1):
    l2[i].append(N-1-l1[i])

ans = 0
for i in range(1,N+1):
    ans += p2[N-1]-1
    ans %= mod
    for j in l2[i]:
        ans -= p2[j]-1
        ans %= mod
ans *= pow(p2[N],mod-2,mod)
ans %= mod
print(ans)


Fixed submission:
from collections import defaultdict,deque
mod = 10**9+7
N = int(input())
g = [[] for _ in range(N)]
d = defaultdict(lambda:-1)
p2 = [1]*(N+1)
p2[1] = pow(2,mod-2,mod)
for i in range(N):
    p2[i+1] = (p2[i]*p2[1])%mod

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

q = deque([[a,b]])
while q:
    i,j = q.pop()
    s = 1
    f = True
    for k in g[j]:
        if k == i:
            continue
        if (j,k) not in d:
            if f:
                q.append([i,j])
            q.append([j,k])
            f = False
        else:
            s += d[(j,k)]
    if f:
        d[(i,j)] = s

q = deque([[b,a]])
while q:
    i,j = q.pop()
    s = 1
    f = True
    for k in g[j]:
        if k == i:
            continue
        if (j,k) not in d:
            if f:
                q.append([i,j])
            q.append([j,k])
            f = False
        else:
            s += d[(j,k)]
    if f:
        d[(i,j)] = s

ans = 1-p2[N]-N*p2[1]

for i in range(N):
    for j in g[i]:
        if j<i:
            continue
        n = max(d[(i,j)],d[(j,i)])
        ans += (1-p2[n])*(1-p2[N-n])
        ans %= mod

print(ans)

Buggy python submission (verdict: Time Limit Exceeded):
import sys
sys.setrecursionlimit(10**6)
from collections import defaultdict
mod = 10**9+7
N = int(input())
g = [[] for _ in range(N)]
d = defaultdict(lambda:-1)
p2 = [1]*(N+1)
p2[1] = pow(2,mod-2,mod)
for i in range(N):
    p2[i+1] = (p2[i]*p2[1])%mod

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

def dfs(i,j):
    if d[(i,j)]!=-1:
        return d[(i,j)]
    res = 1
    for k in g[j]:
        if k==i:
            continue
        res += dfs(j,k)
    d[(i,j)] = res
    return res

for i in range(N):
    if len(g[i])==1:
        dfs(i,g[i][0])
        break

ans = 1-p2[N]-N*p2[1]

for i in range(N):
    for j in g[i]:
        if j<i:
            continue
        n = max(d[(i,j)],d[(j,i)])
        ans += (1-p2[n])*(1-p2[N-n])
        ans %= mod

print(ans)

Fixed submission:
from collections import defaultdict,deque
mod = 10**9+7
N = int(input())
g = [[] for _ in range(N)]
d = defaultdict(lambda:-1)
p2 = [1]*(N+1)
p2[1] = pow(2,mod-2,mod)
for i in range(N):
    p2[i+1] = (p2[i]*p2[1])%mod

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

q = deque([[a,b]])
while q:
    i,j = q.pop()
    s = 1
    f = True
    for k in g[j]:
        if k == i:
            continue
        if (j,k) not in d:
            if f:
                q.append([i,j])
            q.append([j,k])
            f = False
        else:
            s += d[(j,k)]
    if f:
        d[(i,j)] = s

q = deque([[b,a]])
while q:
    i,j = q.pop()
    s = 1
    f = True
    for k in g[j]:
        if k == i:
            continue
        if (j,k) not in d:
            if f:
                q.append([i,j])
            q.append([j,k])
            f = False
        else:
            s += d[(j,k)]
    if f:
        d[(i,j)] = s

ans = 1-p2[N]-N*p2[1]

for i in range(N):
    for j in g[i]:
        if j<i:
            continue
        n = max(d[(i,j)],d[(j,i)])
        ans += (1-p2[n])*(1-p2[N-n])
        ans %= mod

print(ans)

Buggy python submission (verdict: Time Limit Exceeded):
import sys
sys.setrecursionlimit(10**6)
from collections import defaultdict
mod = 10**9+7
N = int(input())
g = [[] for _ in range(N)]
d = defaultdict(lambda:-1)
p2 = [1]*(N+1)
p2[1] = pow(2,mod-2,mod)
for i in range(N):
    p2[i+1] = (p2[i]*p2[1])%mod

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

def dfs(i,j):
    if d[(i,j)]!=-1:
        return d[(i,j)]
    res = 1
    for k in g[j]:
        if k==i:
            continue
        res += dfs(j,k)
    d[(i,j)] = res
    return res

for i in range(N):
    if len(g[i])==1:
        dfs(i,g[i][0])
        break

ans = 1-p2[N]-N*p2[1]

for i in range(N):
    for j in g[i]:
        if j<i:
            continue
        n = max(d[(i,j)],d[(j,i)])
        ans += (1-p2[n])*(1-p2[N-n])
        ans %= mod

print(ans)

Fixed submission:
from collections import defaultdict,deque
mod = 10**9+7
N = int(input())
g = [[] for _ in range(N)]
d = defaultdict(lambda:-1)
p2 = [1]*(N+1)
p2[1] = pow(2,mod-2,mod)
for i in range(N):
    p2[i+1] = (p2[i]*p2[1])%mod

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

q = deque([[a,b]])
while q:
    i,j = q.pop()
    s = 1
    f = True
    for k in g[j]:
        if k == i:
            continue
        if (j,k) not in d:
            if f:
                q.append([i,j])
            q.append([j,k])
            f = False
        else:
            s += d[(j,k)]
    if f:
        d[(i,j)] = s

q = deque([[b,a]])
while q:
    i,j = q.pop()
    s = 1
    f = True
    for k in g[j]:
        if k == i:
            continue
        if (j,k) not in d:
            if f:
                q.append([i,j])
            q.append([j,k])
            f = False
        else:
            s += d[(j,k)]
    if f:
        d[(i,j)] = s

ans = 1-p2[N]-N*p2[1]

for i in range(N):
    for j in g[i]:
        if j<i:
            continue
        n = max(d[(i,j)],d[(j,i)])
        ans += (1-p2[n])*(1-p2[N-n])
        ans %= mod

print(ans)

Buggy python submission (verdict: Time Limit Exceeded):
import sys
sys.setrecursionlimit(10**6)
from collections import defaultdict
mod = 10**9+7
N = int(input())
g = [[] for _ in range(N)]
d = defaultdict(lambda:-1)
p2 = [1]*(N+1)
p2[1] = pow(2,mod-2,mod)
for i in range(N):
    p2[i+1] = (p2[i]*p2[1])%mod

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

def dfs(i,j):
    if d[(i,j)]!=-1:
        return d[(i,j)]
    res = 1
    for k in g[j]:
        if k==i:
            continue
        res += dfs(j,k)
    d[(i,j)] = res
    return res
dfs(a,b)
dfs(b,a)

ans = 1-p2[N]-N*p2[1]

for i in range(N):
    for j in g[i]:
        if j<i:
            continue
        n = max(d[(i,j)],d[(j,i)])
        ans += (1-p2[n])*(1-p2[N-n])
        ans %= mod

print(ans)


Fixed submission:
from collections import defaultdict,deque
mod = 10**9+7
N = int(input())
g = [[] for _ in range(N)]
d = defaultdict(lambda:-1)
p2 = [1]*(N+1)
p2[1] = pow(2,mod-2,mod)
for i in range(N):
    p2[i+1] = (p2[i]*p2[1])%mod

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

q = deque([[a,b]])
while q:
    i,j = q.pop()
    s = 1
    f = True
    for k in g[j]:
        if k == i:
            continue
        if (j,k) not in d:
            if f:
                q.append([i,j])
            q.append([j,k])
            f = False
        else:
            s += d[(j,k)]
    if f:
        d[(i,j)] = s

q = deque([[b,a]])
while q:
    i,j = q.pop()
    s = 1
    f = True
    for k in g[j]:
        if k == i:
            continue
        if (j,k) not in d:
            if f:
                q.append([i,j])
            q.append([j,k])
            f = False
        else:
            s += d[(j,k)]
    if f:
        d[(i,j)] = s

ans = 1-p2[N]-N*p2[1]

for i in range(N):
    for j in g[i]:
        if j<i:
            continue
        n = max(d[(i,j)],d[(j,i)])
        ans += (1-p2[n])*(1-p2[N-n])
        ans %= mod

print(ans)

Buggy python submission (verdict: Time Limit Exceeded):
import sys
sys.setrecursionlimit(10**6)
from collections import defaultdict
mod = 10**9+7
N = int(input())
g = [[] for _ in range(N)]
d = defaultdict(lambda:-1)
p2 = [1]*(N+1)
p2[1] = pow(2,mod-2,mod)
for i in range(N):
    p2[i+1] = (p2[i]*p2[1])%mod

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

def dfs(i,j):
    if d[(i,j)]!=-1:
        return d[(i,j)]
    res = 1
    for k in g[j]:
        if k==i:
            continue
        res += dfs(j,k)
    d[(i,j)] = res
    return res
dfs(a,b)
dfs(b,a)

ans = 1-p2[N]-N*p2[1]

for i in range(N):
    for j in g[i]:
        if j<i:
            continue
        n = max(d[(i,j)],d[(j,i)])
        ans += (1-p2[n])*(1-p2[N-n])
        ans %= mod

print(ans)


Fixed submission:
from collections import defaultdict,deque
mod = 10**9+7
N = int(input())
g = [[] for _ in range(N)]
d = defaultdict(lambda:-1)
p2 = [1]*(N+1)
p2[1] = pow(2,mod-2,mod)
for i in range(N):
    p2[i+1] = (p2[i]*p2[1])%mod

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

q = deque([[a,b]])
while q:
    i,j = q.pop()
    s = 1
    f = True
    for k in g[j]:
        if k == i:
            continue
        if (j,k) not in d:
            if f:
                q.append([i,j])
            q.append([j,k])
            f = False
        else:
            s += d[(j,k)]
    if f:
        d[(i,j)] = s

q = deque([[b,a]])
while q:
    i,j = q.pop()
    s = 1
    f = True
    for k in g[j]:
        if k == i:
            continue
        if (j,k) not in d:
            if f:
                q.append([i,j])
            q.append([j,k])
            f = False
        else:
            s += d[(j,k)]
    if f:
        d[(i,j)] = s

ans = 1-p2[N]-N*p2[1]

for i in range(N):
    for j in g[i]:
        if j<i:
            continue
        n = max(d[(i,j)],d[(j,i)])
        ans += (1-p2[n])*(1-p2[N-n])
        ans %= mod

print(ans)

Buggy python submission (verdict: Runtime Error):
from collections import defaultdict
mod = 10**9+7
N = int(input())
g = [[] for _ in range(N)]
d = defaultdict(lambda:-1)
p2 = [1]*(N+1)
p2[1] = pow(2,mod-2,mod)
for i in range(N):
    p2[i+1] = (p2[i]*p2[1])%mod

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

def dfs(i,j):
    if d[(i,j)]!=-1:
        return d[(i,j)]
    res = 1
    for k in g[j]:
        if k==i:
            continue
        res += dfs(j,k)
    d[(i,j)] = res
    return res
dfs(a,b)
dfs(b,a)

ans = 1-p2[N]-N*p2[1]

for i in range(N):
    for j in g[i]:
        if j<i:
            continue
        n = max(d[(i,j)],d[(j,i)])
        ans += (1-p2[n])*(1-p2[N-n])
        ans %= mod

print(ans)

Fixed submission:
from collections import defaultdict,deque
mod = 10**9+7
N = int(input())
g = [[] for _ in range(N)]
d = defaultdict(lambda:-1)
p2 = [1]*(N+1)
p2[1] = pow(2,mod-2,mod)
for i in range(N):
    p2[i+1] = (p2[i]*p2[1])%mod

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

q = deque([[a,b]])
while q:
    i,j = q.pop()
    s = 1
    f = True
    for k in g[j]:
        if k == i:
            continue
        if (j,k) not in d:
            if f:
                q.append([i,j])
            q.append([j,k])
            f = False
        else:
            s += d[(j,k)]
    if f:
        d[(i,j)] = s

q = deque([[b,a]])
while q:
    i,j = q.pop()
    s = 1
    f = True
    for k in g[j]:
        if k == i:
            continue
        if (j,k) not in d:
            if f:
                q.append([i,j])
            q.append([j,k])
            f = False
        else:
            s += d[(j,k)]
    if f:
        d[(i,j)] = s

ans = 1-p2[N]-N*p2[1]

for i in range(N):
    for j in g[i]:
        if j<i:
            continue
        n = max(d[(i,j)],d[(j,i)])
        ans += (1-p2[n])*(1-p2[N-n])
        ans %= mod

print(ans)