#include <cstdlib>
#include <cctype>
#include <cstring>
#include <cstdio>
#include <cmath>
#include <algorithm>
#include <vector>
#include <string>
#include <iostream>
#include <sstream>
#include <map>
#include <set>
#include <queue>
#include <stack>
#include <fstream>
#include <numeric>
#include <iomanip>
#include <bitset>
#include <list>
#include <stdexcept>
#include <functional>
#include <utility>
#include <ctime>
#include <cassert>
using namespace std;
typedef long long LL;
typedef unsigned long long ULL;
#define MEM(a,b) memset((a),(b),sizeof(a))
const LL INF = 1e9 + 7;
const int N = 1e5 + 10;
int cross(const pair<int, int>& p1, const pair<int, int>& p2)
{
	return p1.first * p2.second - p1.second * p2.first;
}
int main()
{
	//freopen("input.txt", "r", stdin);
	//freopen("output.txt", "w", stdout);
	vector<pair<pair<int, int>, int>> vp;
	int n;
	scanf("%d", &n);
	for (int i = 1; i <= n; i++)
	{
		pair<int, int> p;
		scanf("%d%d", &p.first, &p.second);
		vp.emplace_back(p, i);
	}
	sort(vp.begin(), vp.end());
	printf("%d\n", n / 2);
	for (int i = 0; i + 1 < n; i += 2)
	{
		printf("%d %d\n", vp[i].second, vp[i + 1].second);
	}
	return 0;
}