#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;

vector<set<pair<ll,ll>>> mp(2001);

ll getfood(ll currday, ll minq, ll maxd) {

    //cerr << "\ncurrday=" << currday << "; minq=" << minq << "; maxd=" << maxd;
 
    for(ll i = currday; i <= maxd; i++) {

        if(mp[i].empty()) {
            continue;
        }

        // let's check if there's a minq element

        //cerr << "\ni=" << i << "; mp[i]=" << mp[i];

        pair<ll,ll> srch = {minq, 0};
        //cerr << "\nsrch=" << srch;

        auto o = mp[i].lower_bound({minq, 0});
        if(o == mp[i].end()) {
            //cerr << "\no is the end";
            continue;
        }

        auto h = *o;
        mp[i].erase(o);

        //cerr << "\nh=" << h;

        return h.second;
    }
    
    return -1;
}

void solve() {

	ll n;
	cin >> n;

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

    ll maxtill = 0;

	for(ll i = 1; i <= n; i++) {
		ll till, quality;
		cin >> till >> quality;
		if(quality < 4) {
			continue;
		}

		mp[till].insert({quality,i});

		v[i] = {till, quality};
        
        maxtill = max(till, maxtill);
    }

    vector<pair<ll,ll>> res;

    for(ll i = 1; i <= maxtill; i++) {

        ll a = getfood(i, 4, maxtill);

        if(a == -1) {
            break;
        }

        ll nextmin = 4;

        if(v[a].second == 4) {
            nextmin = 5;
        }

        ll b = getfood(i, nextmin, maxtill);

        if(b == -1) {
            break;
        }

        res.push_back({a, b});
    }

    cout << res.size() << "\n";
    for(const auto & [x, y] : res) {
        cout << x << " " << y << "\n";
    }
}

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

	solve();

	return 0;
}