import sys
sys.setrecursionlimit(5*10**5)
input = sys.stdin.readline
from collections import defaultdict, deque, Counter
from heapq import heappop, heappush
from bisect import bisect_left, bisect_right
from math import gcd
n = int(input())
xs = [[] for i in range(8050)]
for i in range(n):
    x,y = map(int,input().split())
    xs[x].append((y,i))

ans = []
left = []
for x in range(8050):
    xs[x].sort()
    for y,i in xs[x]:
        if not left:
            left.append(i+1)
        else:
            ans.append([i+1,left.pop()])
    
print(len(ans))
for a,b in ans:
    print(a,b)