import sys
input = sys.stdin.readline

from operator import itemgetter

N=int(input())
P=[list(map(int,input().split()))+[i+1] for i in range(N)]

P.sort(key=itemgetter(0))
P.sort(key=itemgetter(1))

ANS=[]

for i in range(0,N,2):
    ANS.append((P[i][2],P[i+1][2]))

print(len(ANS))
for x,y in ANS:
    print(x,y)