#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 int32_t ll;

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

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].push_back({quality,i});

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

    vector<vector<ll>> days(2, vector<ll>(maxtill+1, -1));

    vector<ll> lasti(2, 1);
    // last unfulfilled

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

        for(const auto & [quality, index] : mp[i]) {
            
            if(lasti[1] <= i && quality > 4) {
                days[1][lasti[1]] = index;
                lasti[1]++;
            }
            else if(lasti[0] <= i) {
                days[0][lasti[0]] = index;
                lasti[0]++;
            }
            
        }
    }

    //cerr << "\ndays[0]=" << days[0] << "\ndays[1]=" << days[1];
    lasti[0]--;
    lasti[1]--;

    //cerr << "\nlasti=" << lasti;

    while(lasti[1] > lasti[0] + 1) {

        days[0][lasti[0]+1] = days[1][lasti[1]];
        lasti[0]++;
        lasti[1]--;
    }

    //cerr << "\nlasti=" << lasti;
    //cerr << "\ndays[0]=" << days[0] << "\ndays[1]=" << days[1];

    cout << min(lasti[0], lasti[1]) << "\n";
    for(ll i = 1; i <= min(lasti[0], lasti[1]); i++) {
        cout << days[0][i] << " " << days[1][i] << "\n";
    }

}

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

	solve();

	return 0;
}

/*

. . . 
nem tetszett ez a feladat
ugy erzodik hogy csinalsz valami full random greedy konstrukciot

*/