import bisect import copy import decimal import fractions import heapq import itertools import math import random import sys import time from collections import Counter,deque,defaultdict from functools import lru_cache,reduce from heapq import heappush,heappop,heapify,heappushpop,_heappop_max,_heapify_max def _heappush_max(heap,item): heap.append(item) heapq._siftdown_max(heap, 0, len(heap)-1) def _heappushpop_max(heap, item): if heap and item < heap[0]: item, heap[0] = heap[0], item heapq._siftup_max(heap, 0) return item from math import gcd as GCD read=sys.stdin.read readline=sys.stdin.readline readlines=sys.stdin.readlines write=sys.stdout.write N=int(readline()) X,Y=[],[] for i in range(N): x,y=map(int,readline().split()) X.append(x) Y.append(y) idx=sorted([i for i in range(N)],key=lambda i:(X[i],Y[i])) print(N//2) for z,w in zip(idx[::2],idx[1::2]): print(z+1,w+1)