Buggy python submission (verdict: Runtime Error):
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 roots(self):
      return [i for i, x in enumerate(self.parents) if x < 0]
    
    def group_count(self):
      return len(self.roots())
    
n,m = map(int,input().split())  
uf = UnionFind(n)

for i in range(m):
  a, b = map(int,input().split())
  uf.union(a,b)
print(uf.group_count()-1)

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 roots(self):
      return [i for i, x in enumerate(self.parents) if x < 0]
    
    def group_count(self):
      return len(self.roots())
    
n,m = map(int,input().split())  
uf = UnionFind(n)

for i in range(m):
  a, b = map(int,input().split())
  uf.union(a-1,b-1)
print(uf.group_count()-1)

Buggy python submission (verdict: Runtime Error):
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 roots(self):
      return [i for i, x in enumerate(self.parents) if x < 0]
    
    def group_count(self):
      return len(self.roots())
    
n,m= map(int,input().split())  
uf = UnionFind(n)
ans=0
for i in range(m):
  a, b = map(int,input().split())
  uf.union(a,b)
for i in uf.parents:
  if i < 0:
    ans+=1
print(ans-1)

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 roots(self):
      return [i for i, x in enumerate(self.parents) if x < 0]
    
    def group_count(self):
      return len(self.roots())
    
n,m = map(int,input().split())  
uf = UnionFind(n)

for i in range(m):
  a, b = map(int,input().split())
  uf.union(a-1,b-1)
print(uf.group_count()-1)