#include <bits/stdc++.h>
using namespace std;
#define all(v) (v).begin(),(v).end()
#define pb(a) push_back(a)
#define rep(i, n) for(int i=0;i<n;i++)
#define foa(e, v) for(auto& e : v)
using ll = long long;
const ll MOD7 = 1000000007, MOD998 = 998244353, INF = (1LL << 60);
#define dout(a) cout<<fixed<<setprecision(10)<<a<<endl;

int main() {
    cin.tie(0);
    ios::sync_with_stdio(false);
    ll n;
    cin >> n;
    vector<vector<pair<ll, int>>> dog(10000);
    ll a, b;
    rep(i, n) {
        cin >> a >> b;
        dog[b].push_back({a, i + 1});
    }
    rep(i, 10000) sort(all(dog[i]));
    ll p = -1;
    vector<pair<int, int>> ans;
    rep(i, 10000) {
        int s = dog[i].size();
        if(s == 0) continue;
        if(p == -1) {
            rep(j, s / 2) {
                ans.push_back({dog[i][j * 2].second, dog[i][j * 2 + 1].second});
            }
            if(s&1) p = dog[i].back().second;
        } else {
            ans.push_back({p, dog[i][0].second});
            p = -1;
            rep(j, (s - 1) / 2) {
                ans.push_back({dog[i][j * 2 + 1].second, dog[i][j * 2 + 2].second});
            }
            if(s % 2 == 0) p = dog[i].back().second;
        }
    }
    cout << ans.size() << endl;
    for(auto [x, y] : ans) cout << x << " " << y << endl;
    return 0;
}