// WEBLINK

#include <iostream>
#include <algorithm>
#include <map>
#include <set>
#include <vector>
#include <queue>

using namespace std;

using ll = long long;
using ii = pair<int, int>;
using iii = pair<ii, int>;
using vi = vector<int>;
using vvi = vector<vi>;
using vll = vector<ll>;
using vii = vector<ii>;
using viii = vector<iii>;

#define pb push_back
#define fst first
#define snd second
#define all(x) (x).begin,(x).end()

// ---------- MOD --------------------
const int mod = 998244353;

void add(int & x, int y) {
	x += y;
	if (x < 0) x += mod;
	if (x >= mod) x -= mod;
}
int add2(int x, int y) {
	x += y;
	if (x < 0) x += mod;
	if (x >= mod) x -= mod;
	return x;
}

int mul(int x, int y) {ll r = x; r *= y; return r % mod;}

int power(int x, int p) {
	if (x < 0 or p < 0) return 0;
	int r = 1;
	while (p > 0)
	{
		if (p & 1) r = mul(r, x);
		x = mul(x, x); p /= 2;
	} return r;
}
int inv(int a) { return power(a, mod - 2); }

template<typename T, typename T1>T amax(T &a, T1 b) {if (b > a)a = b; return a;}
template<typename T, typename T1>T amin(T &a, T1 b) {if (b < a)a = b; return a;}

// ---------- DEBUG --------------------


//#define ON_PC
#ifdef ON_PC
#include </Users/karanaggarwal/code/debug.h>
#else
#define deb(x...)
#endif


///////////////////////////////////

string f()
{
	int x; cin >> x;

	if (x >= 12 or x < 3) return "winter";
	if (x >= 3 and  x < 6) return "spring";
	if (x >= 6 and  x < 9) return "summer";
	return "fall";
}

int main()
{
	ios_base::sync_with_stdio(false);
	cin.tie(NULL);


	int n; cin >> n;
	viii A(n);

	int i = 0;
	for (auto & x : A)
	{
		x.snd = i++;
		cin >> x.fst.fst;
		cin >> x.fst.snd;
	}

	sort(A.begin(), A.end());

	cout << n / 2 << endl;

	for (int i = 0; i + 1 < n; i += 2)
	{
		cout << A[i].second + 1 << " " << A[i + 1].second + 1 << endl;
	}

	return 0;
}