#int(input())
#map(int, input().split())
#list(map(int, input().split()))

import time
import random
from math import floor, log10

N = int(input())
u = [0] * (2 * N)
for i in range(N):
    a, b = map(int, input().split())
    u[i] = a
    u[i+N] = b

ut = time.time()

def get_score(u):
    c = 5 * 10 ** 17
    v1 = abs(u[0] - c)
    v2 = abs(u[N] - c)
    s = floor(2 * 10 ** 6 - 10 ** 5 * log10(max(v1, v2) + 1))
    return s

M = 50
ans = [0] * (2 * M)

for i in range(2*M):
    ans[i] = random.randint(0, N-1)
# for i in range(10):
#     ans[i] = 0

def f(a):
    nu = list(u)
    for i in range(M):
        if a[i] == a[i+M]:
            continue
        x = a[i]
        y = a[i+M]
        nu[x] = floor((nu[x]+nu[y]) / 2)
        nu[y] = nu[x]
        nu[x+N] = floor((nu[x+N]+nu[y+N]) / 2)
        nu[y+N] = nu[y+N]

    return nu

nows = get_score(f(ans))
count = 0

while time.time() - ut < 0.9:
    nans = list(ans)
    t = random.randint(0, 2*M-1)
    n = random.randint(0, N-1)
    nans[t] = n
    s = get_score(f(nans))
    if s > nows:
        ans = list(nans)
        # print(s, nows)
        nows = s
        count += 1

fa = f(ans)
# print(count)
# print(fa[:3], fa[N:N+3])

def stdout(ans):
    l = []
    for i in range(M):
        if ans[i] == ans[i+M]:
            continue
        l.append([ans[i], ans[i+M]])
    print(len(l))
    for x in l:
        print(*x)

stdout(ans)