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 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())
un = UnionFind(n)
for _ in range(m):
    a, b = map(int, input().split())
    un.union(a, b)
print(un.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 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())
un = UnionFind(n)
for _ in range(m):
    a, b = map(int, input().split())
    a -= 1
    b -= 1
    un.union(a, b)
print(un.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 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())
un = UnionFind(n)
for _ in range(m):
    a, b = map(int, input().split())
    un.union(a, b)
print(un.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 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())
un = UnionFind(n)
for _ in range(m):
    a, b = map(int, input().split())
    a -= 1
    b -= 1
    un.union(a, b)
print(un.group_count() - 1)