/* -*- coding: utf-8 -*-
 *
 * 2353.cc:  No.2353 Guardian Dogs in Spring - yukicoder
 */

#include<cstdio>
#include<vector>
#include<algorithm>
 
using namespace std;

/* constant */

const int MAX_N = 8000;

/* typedef */

typedef vector<int> vi;

/* global variables */

vi ps[MAX_N];

/* subroutines */

/* main */

int main() {
  int n;
  scanf("%d", &n);
  for (int i = 0; i < n; i++) {
    int x, y;
    scanf("%d%d", &x, &y);
    ps[i] = { x, y, i };
  }
  sort(ps, ps + n);

  printf("%d\n", n / 2);
  for (int i = 0; i + 1 < n; i += 2)
    printf("%d %d\n", ps[i][2] + 1, ps[i + 1][2] + 1);

  return 0;
}