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)