#include <iostream>
#include <algorithm> // for sort, mainly
#include <vector>
#include <map>
#include <set>
#include <cmath>
#include <array>
#include <string>
#include <cstdio>
#include <iterator>
#include <unordered_set>
#include <cstdint> // for int64_t, int32_t, etc
#include <queue>
#include <stack>
#include <deque>
#include <numeric> // gcd, lcm
#include <fstream>
#include <bitset> // for bitset
#include <iomanip>
#include <cassert> // for set with custom ordering
#include <type_traits> // for set with custom ordering
#include <utility> // for swap, forward, etc
using namespace std;

#pragma GCC optimize("O2")
// #pragma GCC optimize("O1","O2","O3","Ofast","unroll-loops")
// #pragma GCC target("sse","sse2","sse3","sse4.1","sse4.2","avx","avx2","fma")

template<typename A, typename B> ostream& operator<<(ostream &os, const pair<A, B> &p) { return os << '(' << p.first << ", " << p.second << ')'; }
template<typename T_container, typename T = typename enable_if<!is_same<T_container, string>::value, typename T_container::value_type>::type> ostream& operator<<(ostream &os, const T_container &v) { os << '{'; string sep; for (const T &x : v) os << sep << x, sep = ", "; return os << '}'; }
void dbg_out() { cout << endl; }
template<typename Head, typename... Tail> void dbg_out(Head H, Tail... T) { cout << ' ' << H; dbg_out(T...); }
#ifdef LOCAL
#define dbg(...) cout << "(" << #__VA_ARGS__ << "):", dbg_out(__VA_ARGS__)
#else
#define dbg(...)
#endif

/*

notes:

int64_t
stoi(string s) -> string to int
to_string() -> int (or else) to string

vector declaration:
vector<ll> v(n, 0)
vector<vector<ll>> v(n, vector<ll>(n, 0));

{if statement} ? {truth value} : {false value}

#ifdef LOCAL
    freopen("in.txt","r",stdin);
    freopen("out.txt","w",stdout);
#endif

std::lcm(ll a, ll b), std::gcd(int a, int b)

cout << fixed << setprecision(n);

set with custom ordering
set<ll, decltype(&cmp)> qu(cmp);

*/

typedef int64_t ll;

set<pair<ll,ll>> s;

map<pair<ll,ll>, queue<ll>> mp;

ll getfood(ll currday, ll minq) {

	auto it = s.lower_bound({currday, minq});
	if(it == s.end()) {
		return -1;
	}
	
	ll ret = mp[*it].front();
	mp[*it].pop();
	s.erase(it);

	return ret;
}

void solve() {

	ll n;
	cin >> n;

	vector<pair<ll,ll>> v(n+1, {0,0});

	for(ll i = 1; i <= n; i++) {
		ll till, quality;
		cin >> till >> quality;
		if(quality < 4) {
			continue;
		}
		s.insert({till, quality});
		mp[{till, quality}].push(i);
		v[i] = {till, quality};
	}

	cerr << "\nn=" << n;
	cerr << "\nv=" << v;
	cerr << "\ns=" << s;
	//cerr << "\nmp=" << mp;

	ll d = 1;
	vector<ll> res;
	while(1) {
		
		auto fst = getfood(d, 4);

		if(fst == -1) {
			// can't find any more
			d--;
			break;
		}

		ll secfq = 4;
		if(v[fst].second == 4) {
			secfq = 5;
		}

		auto sct = getfood(d, secfq);
		if(sct == -1) {
			d--;
			break;
		}

		res.push_back(fst);
		res.push_back(sct);

		d++;
	}

	cout << d << "\n";
	for(ll i = 0; i < res.size(); i+=2) {
		cout << res[i] << " " << res[i+1] << "\n";
	}

}

int main()
{
	std::ios_base::sync_with_stdio(false);
	//cin.tie(nullptr);
	//cout.tie(nullptr);

	solve();

	return 0;
}