N = int(input())
inu = []
for i in range(N):
	x,y = map(int,input().split())
	inu.append((x,y,i + 1))
	
inu.sort()
print(N // 2)
for _ in range(N // 2):
	x,y,i = inu.pop()
	xx,yy,ii = inu.pop()
	print(i,ii)