import sys
input = lambda: sys.stdin.readline().rstrip()
ii = lambda: int(input())
mi = lambda: map(int, input().split())
li = lambda: list(mi())
INF = 2**63-1
mod = 998244353
from math import gcd
n = ii()

assert 1 <= n <= 8000
XY = [tuple(li()) for _ in range(n)]

for x, y in XY:
    assert 0 <= x <= 8000 and 0 <= y <= 8000

assert len(set(XY)) == n

p = list(range(1, n + 1))
p.sort(key = lambda x: XY[x - 1])

print(n // 2)

for i in range(n // 2):
    print(p[2 * i], p[2 * i + 1])