N = int(input())

Y = []
posi = []
for i in range(N):
    x,y = map(int, input().split())
    Y.append(y)
    posi.append((i,x,y))
posi.sort(key=lambda x:x[1])

d = {num:i for i,num in enumerate(sorted(list(set(Y))))}
p = [list() for _ in range(N)]
for i,x,y in posi:
    p[d[y]].append((i,x))

res = []
m = []
for i in range(len(p)):
    if len(p[i])>=3:
        cnt = len(p[i])//2
        while cnt:
            id1,_ = p[i].pop()
            id2,_ = p[i].pop()
            res.append((id1+1,id2+1))
            cnt -= 1
    while p[i]:
        id,_ = p[i].pop()
        m.append(id+1)

cnt = len(m)//2
while cnt:
    id1 = m.pop()
    id2 = m.pop()
    res.append((id1,id2))
    cnt -= 1
print(len(res))
for a,b in res:
    print(a,b)