import random
N=int(input())
a=random.randrange(10000,100000000)
b=random.randrange(10000,100000000)
p=[tuple(map(int,input().split())) for _ in range(N)]
l=[(i,a*p[i][0]+b*p[i][1]) for i in range(N)]
l.sort(key=lambda tup:tup[1])
print(N//2)
for i in range(N//2):
    print(l[2*i][0]+1,l[2*i+1][0]+1)