n=int(input())
p=[tuple(map(int,input().split())) for i in range(n)]
L=8000
q=[[] for i in range(L+1)]
for i in range(n):
  x,y=p[i]
  q[y]+=[(x,i)]
a=[]
for i in range(L+1):
  q[i].sort()
  for _,j in q[i]:
    a+=[j+1]
print(n//2)
for i in range(n//2):
  print(a[i*2],a[i*2+1])