Buggy python submission (verdict: Time Limit Exceeded):

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())
uf = UnionFind(n)
fr = []
# G = [[] for _ in range(n)]
for i in range(m):
    a,b = map(int, input().split())
    a -= 1
    b -= 1
    uf.union(a,b)
    fr.append((a,b))
c = [0]*n
for i in range(n):
    c[i] = tuple(uf.members(i))

# print(c)
sc = set(c)
sc = list(sc)
lenc = []
for ss in sc:
    lenc.append(len(ss))
lenc = set(lenc)
lenc = list(lenc)
lenc.sort(reverse=1)
if len(lenc) == 1:
    print(lenc[0])
else:
    ans = lenc[1]
    ans += lenc[0]-lenc[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())
uf = UnionFind(n)
fr = []
# G = [[] for _ in range(n)]
for i in range(m):
    a,b = map(int, input().split())
    a -= 1
    b -= 1
    uf.union(a,b)
    fr.append((a,b))
c = []
# tmp_s = set()
for i in range(n):
    c.append(uf.size(i))
    # c[i] = tuple(uf.members(i))
# print(c)
c = set(c)
c = list(c)
c.sort(reverse=1)
if len(c) == 1:
    print(c[0])
else:
    ans = c[1]
    ans += c[0]-c[1]
    print(ans)
# print(c)
# sc = set(c)
# sc = list(sc)
# lenc = []
# for ss in sc:
#     lenc.append(len(ss))
# lenc = set(lenc)
# lenc = list(lenc)
# lenc.sort(reverse=1)
# if len(lenc) == 1:
#     print(lenc[0])
# else:
#     ans = lenc[1]
#     ans += lenc[0]-lenc[1]
#     print(ans)