n = int(input())
XY = []
for i in range(n):
	x,y = map(int,input().split())
	XY.append([x,y,i+1])
	
	
XY.sort()
print(n//2)
for i in range(n//2):
	a = XY[2*i][2]
	b = XY[2*i+1][2]
	print(a,b)