Buggy python submission (verdict: Time Limit Exceeded):
def dfs(tree, v, tv, p, es):
    if v == tv:
        return True
    for e in tree[v]:
        if e[0] == p:
            continue
        if dfs(tree, e[0], tv, v, es):
            es.append(e[1])
            return True
    return False


def get_bit_count(bit):
    return format(bit, 'b').count('1')


def main():
    n = int(input())
    tree = [[] for i in range(n)]
    for i in range(n-1):
        a, b = map(int, input().split())
        a -= 1
        b -= 1
        tree[a].append((b, i))
        tree[b].append((a, i))
    m = int(input())
    eset = [0]*m
    for i in range(m):
        u, v = map(int, input().split())
        es = []
        dfs(tree, u-1, v-1, -1, es)
        for e in es:
            eset[i] |= 1 << e

    ans = 0
    for i in range(1 << m):
        mask = 0
        for j in range(m):
            if i >> j & 1:
                mask |= eset[j]
        white = get_bit_count(mask)
        now = 1 << (n-1-white)
        if get_bit_count(i) % 2 == 0:
            ans += now
        else:
            ans -= now
    print(ans)


main()


Fixed submission:
def dfs(tree, v, tv, p, es):
    if v == tv:
        return True
    for e in tree[v]:
        if e[0] == p:
            continue
        if dfs(tree, e[0], tv, v, es):
            es.append(e[1])
            return True
    return False


def main():
    n = int(input())
    tree = [[] for i in range(n)]
    for i in range(n-1):
        a, b = map(int, input().split())
        a -= 1
        b -= 1
        tree[a].append((b, i))
        tree[b].append((a, i))
    m = int(input())
    eset = [0]*m
    for i in range(m):
        u, v = map(int, input().split())
        es = []
        dfs(tree, u-1, v-1, -1, es)
        for e in es:
            eset[i] |= 1 << e

    ans = 0
    for i in range(1 << m):
        mask = 0
        for j in range(m):
            if i >> j & 1:
                mask |= eset[j]
        white = bin(mask).count('1')
        now = 1 << (n-1-white)
        if bin(i).count('1') % 2 == 0:
            ans += now
        else:
            ans -= now
    print(ans)


main()

Buggy python submission (verdict: Time Limit Exceeded):
def dfs(tree, v, tv, p, es):
    if v == tv:
        return True
    for e in tree[v]:
        if e[0] == p:
            continue
        if dfs(tree, e[0], tv, v, es):
            es.append(e[1])
            return True
    return False


def get_bit_count(bit):
    return format(bit, 'b').count('1')


def main():
    n = int(input())
    tree = {}
    for i in range(n-1):
        a, b = map(int, input().split())
        a -= 1
        b -= 1
        if a not in tree:
            tree[a] = []
        if b not in tree:
            tree[b] = []
        tree[a].append((b, i))
        tree[b].append((a, i))
    m = int(input())
    eset = [0]*m
    for i in range(m):
        u, v = map(int, input().split())
        es = []
        dfs(tree, u-1, v-1, -1, es)
        for e in es:
            eset[i] |= 1 << e

    ans = 0
    for i in range(1 << m):
        mask = 0
        for j in range(m):
            if i >> j & 1:
                mask |= eset[j]
        white = get_bit_count(mask)
        now = 1 << (n-1-white)
        if get_bit_count(i) % 2 == 0:
            ans += now
        else:
            ans -= now
    print(ans)


main()


Fixed submission:
def dfs(tree, v, tv, p, es):
    if v == tv:
        return True
    for e in tree[v]:
        if e[0] == p:
            continue
        if dfs(tree, e[0], tv, v, es):
            es.append(e[1])
            return True
    return False


def main():
    n = int(input())
    tree = [[] for i in range(n)]
    for i in range(n-1):
        a, b = map(int, input().split())
        a -= 1
        b -= 1
        tree[a].append((b, i))
        tree[b].append((a, i))
    m = int(input())
    eset = [0]*m
    for i in range(m):
        u, v = map(int, input().split())
        es = []
        dfs(tree, u-1, v-1, -1, es)
        for e in es:
            eset[i] |= 1 << e

    ans = 0
    for i in range(1 << m):
        mask = 0
        for j in range(m):
            if i >> j & 1:
                mask |= eset[j]
        white = bin(mask).count('1')
        now = 1 << (n-1-white)
        if bin(i).count('1') % 2 == 0:
            ans += now
        else:
            ans -= now
    print(ans)


main()