import sys
from sys import stdin

N = int(stdin.readline())

dic = {}
xlis = []

for i in range(N):

    x,y = map(int,stdin.readline().split())

    if x not in dic:
        dic[x] = []
        xlis.append(x)
    
    dic[x].append( (y,i+1) )

wait = None
ans = []

xlis.sort()

for x in xlis:

    dic[x].sort()

    for y,i in dic[x]:
        if wait != None:
            ans.append( (wait,i) )
            wait = None
        else:
            wait = i

print (len(ans))
for i,j in ans:
    print (i,j)