Buggy python submission (verdict: Runtime Error):
n = int(input())
dx = defaultdict(list)
dy = defaultdict(list)
pts = set()
for _ in range(n):
    x, y = map(int, input().split())
    dx[x].append(y)
    dy[y].append(x)
    pts.add((x, y))

vx, vy = set(), set()
cnt = 0
for x, y in pts:
    if x in vx or y in vy: continue
    sx, sy = [x], [y]
    xs, ys = set(), set()
    while sx or sy:
        while sx:
            x = sx.pop()
            if x not in xs:
                xs.add(x)
                for y in dx[x]:
                    if y not in ys:
                        sy.append(y)
        while sy:
            y = sy.pop()
            if y not in ys:
                ys.add(y)
                for x in dy[y]:
                    if x not in xs:
                        sx.append(x)
    vx.update(xs)
    vy.update(ys)
    cnt += len(xs) * len(ys)
print(cnt - len(pts))

Fixed submission:
from collections import defaultdict
n = int(input())
dx = defaultdict(list)
dy = defaultdict(list)
pts = set()
for _ in range(n):
    x, y = map(int, input().split())
    dx[x].append(y)
    dy[y].append(x)
    pts.add((x, y))

vx, vy = set(), set()
cnt = 0
for x, y in pts:
    if x in vx or y in vy: continue
    sx, sy = [x], [y]
    xs, ys = set(), set()
    while sx or sy:
        while sx:
            x = sx.pop()
            if x not in xs:
                xs.add(x)
                for y in dx[x]:
                    if y not in ys:
                        sy.append(y)
        while sy:
            y = sy.pop()
            if y not in ys:
                ys.add(y)
                for x in dy[y]:
                    if x not in xs:
                        sx.append(x)
    vx.update(xs)
    vy.update(ys)
    cnt += len(xs) * len(ys)
print(cnt - len(pts))

Buggy python submission (verdict: Runtime Error):
n = int(input())
dot = {}
dotset = set()
for i in range(n):
    x, y = map(int, input().split())
    dot[i] = (x, y)
    dotset.add((x, y))

v = set()

cnt = 0
for i, f in dot:
    a, b = f
    for j, g in dot:
        if i == j: continue
        c, d = g
        if a == c or b == d: continue
        f1 = (a, d) in dotset
        f2 = (c, b) in dotset
        if f1 and not f2:
            cnt += 1
            dotset.add((c, b))
        if not f1 and f2:
            cnt += 1
            dotset.add((a, d))
print(cnt)

Fixed submission:
from collections import defaultdict
n = int(input())
dx = defaultdict(list)
dy = defaultdict(list)
pts = set()
for _ in range(n):
    x, y = map(int, input().split())
    dx[x].append(y)
    dy[y].append(x)
    pts.add((x, y))

vx, vy = set(), set()
cnt = 0
for x, y in pts:
    if x in vx or y in vy: continue
    sx, sy = [x], [y]
    xs, ys = set(), set()
    while sx or sy:
        while sx:
            x = sx.pop()
            if x not in xs:
                xs.add(x)
                for y in dx[x]:
                    if y not in ys:
                        sy.append(y)
        while sy:
            y = sy.pop()
            if y not in ys:
                ys.add(y)
                for x in dy[y]:
                    if x not in xs:
                        sx.append(x)
    vx.update(xs)
    vy.update(ys)
    cnt += len(xs) * len(ys)
print(cnt - len(pts))