Buggy python submission (verdict: Runtime Error):
#入力:N,M(int:整数)
def input2():
	return map(int,input().split())

#入力:[n1,n2,...nk](int:整数配列)
def input_array():
	return list(map(int,input().split()))
  
n,m=input2()
AB=[input_array() for _ in range(m)]


uf = UnionFind(n)
for ab in AB:
	a=ab[0]
	b=ab[1]
	uf.union(a-1,b-1)

ans=0
for i in uf.parents:
	if i < 0:
		ans+=1
print(ans-1)

Fixed submission:
#入力:N,M(int:整数)
def input2():
	return map(int,input().split())

#入力:[n1,n2,...nk](int:整数配列)
def input_array():
	return list(map(int,input().split()))

class UnionFind():
	"""docstring for UnionFind"""
	def __init__(self, n):
		self.parents = [-1]*n

	def find(self,x):
		if self.parents[x] < 0: #自分が親である時
			return x
		else: #自分が子供である時
			self.parents[x]=self.find(self.parents[x])
			return self.parents[x]

	def union(self,x,y):
		# 各要素の親要素を返す
		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
		# print(self.parents)
n,m=input2()
AB=[input_array() for _ in range(m)]


uf = UnionFind(n)
for ab in AB:
	a=ab[0]
	b=ab[1]
	uf.union(a-1,b-1)

ans=0
for i in uf.parents:
	if i < 0:
		ans+=1
print(ans-1)

Buggy python submission (verdict: Time Limit Exceeded):
#入力:N,M(int:整数)
def input2():
	return map(int,input().split())

#入力:[n1,n2,...nk](int:整数配列)
def input_array():
	return list(map(int,input().split()))
  
class UnionFind():
	"""docstring for UnionFind"""
	def __init__(self, n):
		self.parents = [-1]*n

	def find(self,x):
		if self.parents[x] < 0: #自分が親である時
			return x
		else: #自分が子供である時
			self.parents[x]=self.find(self.parents[x])
			return self.parents[x]

	def union(self,x,y):
		# 各要素の親要素を返す
		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
        
n,m=input2()
AB=[input_array() for _ in range(m)]
D=[[0]*n for _ in range(n)]

uf = UnionFind(n)
for ab in AB:
	a=ab[0]
	b=ab[1]
	uf.union(a-1,b-1)

ans=0
for i in uf.parents:
	if i < 0:
		ans+=1
print(ans-1)


Fixed submission:
#入力:N,M(int:整数)
def input2():
	return map(int,input().split())

#入力:[n1,n2,...nk](int:整数配列)
def input_array():
	return list(map(int,input().split()))

class UnionFind():
	"""docstring for UnionFind"""
	def __init__(self, n):
		self.parents = [-1]*n

	def find(self,x):
		if self.parents[x] < 0: #自分が親である時
			return x
		else: #自分が子供である時
			self.parents[x]=self.find(self.parents[x])
			return self.parents[x]

	def union(self,x,y):
		# 各要素の親要素を返す
		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
		# print(self.parents)
n,m=input2()
AB=[input_array() for _ in range(m)]


uf = UnionFind(n)
for ab in AB:
	a=ab[0]
	b=ab[1]
	uf.union(a-1,b-1)

ans=0
for i in uf.parents:
	if i < 0:
		ans+=1
print(ans-1)

Buggy python submission (verdict: Time Limit Exceeded):
#入力:N,M(int:整数)
def input2():
	return map(int,input().split())

#入力:[n1,n2,...nk](int:整数配列)
def input_array():
	return list(map(int,input().split()))
  
class UnionFind():
	"""docstring for UnionFind"""
	def __init__(self, n):
		self.parents = [-1]*n

	def find(self,x):
		if self.parents[x] < 0: #自分が親である時
			return x
		else: #自分が子供である時
			self.parents[x]=self.find(self.parents[x])
			return self.parents[x]

	def union(self,x,y):
		# 各要素の親要素を返す
		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
        
n,m=input2()
AB=[input_array() for _ in range(m)]
D=[[0]*n for _ in range(n)]

uf = UnionFind(n)
for ab in AB:
	a=ab[0]
	b=ab[1]
	uf.union(a-1,b-1)

ans=0
for i in range(n):
	tmp=uf.find(i)
	if tmp!=0:
		ans+=1
print(ans)

Fixed submission:
#入力:N,M(int:整数)
def input2():
	return map(int,input().split())

#入力:[n1,n2,...nk](int:整数配列)
def input_array():
	return list(map(int,input().split()))

class UnionFind():
	"""docstring for UnionFind"""
	def __init__(self, n):
		self.parents = [-1]*n

	def find(self,x):
		if self.parents[x] < 0: #自分が親である時
			return x
		else: #自分が子供である時
			self.parents[x]=self.find(self.parents[x])
			return self.parents[x]

	def union(self,x,y):
		# 各要素の親要素を返す
		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
		# print(self.parents)
n,m=input2()
AB=[input_array() for _ in range(m)]


uf = UnionFind(n)
for ab in AB:
	a=ab[0]
	b=ab[1]
	uf.union(a-1,b-1)

ans=0
for i in uf.parents:
	if i < 0:
		ans+=1
print(ans-1)

Buggy python submission (verdict: Time Limit Exceeded):
from scipy.sparse.csgraph import connected_components
from scipy.sparse import csr_matrix

#入力:N,M(int:整数)
def input2():
	return map(int,input().split())

#入力:[n1,n2,...nk](int:整数配列)
def input_array():
	return list(map(int,input().split()))
  
n,m=input2()
AB=[input_array() for _ in range(m)]
D=[[0]*n for _ in range(n)]

for ab in AB:
	a=ab[0]
	b=ab[1]
	D[a-1][b-1]=1

csr=csr_matrix(D)

ans=connected_components(csr, return_labels=False) -1
print(ans)

Fixed submission:
#入力:N,M(int:整数)
def input2():
	return map(int,input().split())

#入力:[n1,n2,...nk](int:整数配列)
def input_array():
	return list(map(int,input().split()))

class UnionFind():
	"""docstring for UnionFind"""
	def __init__(self, n):
		self.parents = [-1]*n

	def find(self,x):
		if self.parents[x] < 0: #自分が親である時
			return x
		else: #自分が子供である時
			self.parents[x]=self.find(self.parents[x])
			return self.parents[x]

	def union(self,x,y):
		# 各要素の親要素を返す
		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
		# print(self.parents)
n,m=input2()
AB=[input_array() for _ in range(m)]


uf = UnionFind(n)
for ab in AB:
	a=ab[0]
	b=ab[1]
	uf.union(a-1,b-1)

ans=0
for i in uf.parents:
	if i < 0:
		ans+=1
print(ans-1)