N=int(input())
Points=[list(map(int,input().split()))+[i+1] for i in range(N)]
Points.sort(key=lambda x:x[1])
Points.sort(key=lambda x:x[0])
res=[]
ans=N//2
from collections import deque
q=deque(Points)
while len(q)>=2:
    _,_,i=q.popleft()
    _,_,j=q.popleft()
    res.append((i,j))
print(ans)
for c in res:
    print(*c)