Buggy python submission (verdict: Runtime Error):
class UnionFind:
  def __init__(self, n):
    self.parents = list(range(n))
    self.ranks = [1 for _ in range(n)]
  def parent(self, x):
    if x == self.parents[x]:
      return x
    else:
      p = self.parent(self.parents[x])
      self.parents[x] = p
      return p
  def union(self, x, y):
    host = x if self.ranks[x] < self.ranks[y] else y
    guest = x+y-host
    self.ranks[host] += self.ranks[guest]
    self.parents[self.parent(guest)] = host
  def find(self, x, y):
    return self.parent(x)==self.parent(y)
  def numtrees(self):
    return len(set(self.parents))

N, M = map(int, input().split())
E = []
for _ in range(M):
  a, b = map(int, input().split())
  E.append((a-1, b-1))
ans = 0
r = 0
while r < M:
  uf = UnionFind(N)
  i = 0
  while i < M:
    if i != r:
      a, b = E[i]
      uf.union(a, b)
    i += 1
  i = 0
  while i < M:
    _ = uf.parent(i)
    i += 1
  if uf.numtrees() > 1:
    ans += 1
  r += 1
print(ans)

Fixed submission:
class UnionFind():
    def __init__(self, n):
        self.n = n
        self.parents = [-1] * n

    def find(self, x):
        if self.parents[x] < 0:
            return x
        else:
            self.parents[x] = self.find(self.parents[x])
            return self.parents[x]

    def union(self, x, y):
        x = self.find(x)
        y = self.find(y)

        if x == y:
            return

        if self.parents[x] > self.parents[y]:
            x, y = y, x

        self.parents[x] += self.parents[y]
        self.parents[y] = x

    def size(self, x):
        return -self.parents[self.find(x)]

    def same(self, x, y):
        return self.find(x) == self.find(y)

    def members(self, x):
        root = self.find(x)
        return [i for i in range(self.n) if self.find(i) == root]

    def roots(self):
        return [i for i, x in enumerate(self.parents) if x < 0]

    def group_count(self):
        return len(self.roots())

    def all_group_members(self):
        return {r: self.members(r) for r in self.roots()}

    def __str__(self):
        return '\n'.join('{}: {}'.format(r, self.members(r)) for r in self.roots())

N, M = map(int, input().split())
E = []
for _ in range(M):
  a, b = map(int, input().split())
  E.append((a-1, b-1))
ans = 0
r = 0
while r < M:
  uf = UnionFind(N)
  i = 0
  while i < M:
    if i != r:
      a, b = E[i]
      uf.union(a, b)
    i += 1
  i = 0
  while i < N:
    _ = uf.find(i)
    i += 1
  if uf.group_count() > 1:
    ans += 1
  r += 1
print(ans)

Buggy python submission (verdict: Runtime Error):
class UnionFind:
  def __init__(self, n):
    self.parents = list(range(n))
    self.ranks = [1 for _ in range(n)]
  def parent(self, x):
    if x == self.parents[x]:
      return x
    else:
      p = self.parent(self.parents[x])
      self.parents[x] = p
      return p
  def union(self, x, y):
    host = x if self.ranks[x] < self.ranks[y] else y
    guest = x+y-host
    self.ranks[host] += self.ranks[guest]
    self.parents[self.parent(guest)] = host
  def find(self, x, y):
    return self.parent(x)==self.parent(y)
  def numtrees(self):
    return len(set(self.parents))

N, M = map(int, input().split())
E = []
for _ in range(M):
  a, b = map(int, input().split())
  E.append((a-1, b-1))
ans = 0
r = 0
while r < M:
  uf = UnionFind(N)
  i = 0
  while i < M:
    if i != r:
      a, b = E[i]
      uf.union(a, b)
    i += 1
  i = 0
  while i < N:
    _ = uf.parent(i)
    i += 1
  if uf.numtrees() > 1:
    ans += 1
  r += 1
print(ans)

Fixed submission:
class UnionFind():
    def __init__(self, n):
        self.n = n
        self.parents = [-1] * n

    def find(self, x):
        if self.parents[x] < 0:
            return x
        else:
            self.parents[x] = self.find(self.parents[x])
            return self.parents[x]

    def union(self, x, y):
        x = self.find(x)
        y = self.find(y)

        if x == y:
            return

        if self.parents[x] > self.parents[y]:
            x, y = y, x

        self.parents[x] += self.parents[y]
        self.parents[y] = x

    def size(self, x):
        return -self.parents[self.find(x)]

    def same(self, x, y):
        return self.find(x) == self.find(y)

    def members(self, x):
        root = self.find(x)
        return [i for i in range(self.n) if self.find(i) == root]

    def roots(self):
        return [i for i, x in enumerate(self.parents) if x < 0]

    def group_count(self):
        return len(self.roots())

    def all_group_members(self):
        return {r: self.members(r) for r in self.roots()}

    def __str__(self):
        return '\n'.join('{}: {}'.format(r, self.members(r)) for r in self.roots())

N, M = map(int, input().split())
E = []
for _ in range(M):
  a, b = map(int, input().split())
  E.append((a-1, b-1))
ans = 0
r = 0
while r < M:
  uf = UnionFind(N)
  i = 0
  while i < M:
    if i != r:
      a, b = E[i]
      uf.union(a, b)
    i += 1
  i = 0
  while i < N:
    _ = uf.find(i)
    i += 1
  if uf.group_count() > 1:
    ans += 1
  r += 1
print(ans)

Buggy python submission (verdict: Runtime Error):
class UnionFind:
  def __init__(self, n):
    self.parents = list(range(n))
    self.ranks = [1 for _ in range(n)]
  def parent(self, x):
    if x == self.parents[x]:
      return x
    else:
      p = self.parent(self.parents[x])
      self.parents[x] = p
      return p
  def union(self, x, y):
    host = x if self.ranks[x] < self.ranks[y] else y
    guest = x+y-host
    self.ranks[host] += self.ranks[guest]
    self.parents[self.parent(guest)] = host
  def find(self, x, y):
    return self.parent(x)==self.parent(y)
  def numtrees(self):
    return len(set(self.parents))

N, M = map(int, input().split())
E = []
for _ in range(M):
  a, b = map(int, input().split())
  E.append((a-1, b-1))
ans = 0
r = 0
while r < N:
  uf = UnionFind(N)
  i = 0
  while i < M:
    if i != r:
      a, b = E[i]
      uf.union(a, b)
    i += 1
  i = 0
  while i < N:
    _ = uf.parent(i)
    i += 1
  if uf.numtrees() > 1:
    ans += 1
  r += 1
print(ans)

Fixed submission:
class UnionFind():
    def __init__(self, n):
        self.n = n
        self.parents = [-1] * n

    def find(self, x):
        if self.parents[x] < 0:
            return x
        else:
            self.parents[x] = self.find(self.parents[x])
            return self.parents[x]

    def union(self, x, y):
        x = self.find(x)
        y = self.find(y)

        if x == y:
            return

        if self.parents[x] > self.parents[y]:
            x, y = y, x

        self.parents[x] += self.parents[y]
        self.parents[y] = x

    def size(self, x):
        return -self.parents[self.find(x)]

    def same(self, x, y):
        return self.find(x) == self.find(y)

    def members(self, x):
        root = self.find(x)
        return [i for i in range(self.n) if self.find(i) == root]

    def roots(self):
        return [i for i, x in enumerate(self.parents) if x < 0]

    def group_count(self):
        return len(self.roots())

    def all_group_members(self):
        return {r: self.members(r) for r in self.roots()}

    def __str__(self):
        return '\n'.join('{}: {}'.format(r, self.members(r)) for r in self.roots())

N, M = map(int, input().split())
E = []
for _ in range(M):
  a, b = map(int, input().split())
  E.append((a-1, b-1))
ans = 0
r = 0
while r < M:
  uf = UnionFind(N)
  i = 0
  while i < M:
    if i != r:
      a, b = E[i]
      uf.union(a, b)
    i += 1
  i = 0
  while i < N:
    _ = uf.find(i)
    i += 1
  if uf.group_count() > 1:
    ans += 1
  r += 1
print(ans)