#include<bits/stdc++.h>
using namespace std;

#define all(v) v.begin(),v.end()
using ll = long long;
using ull = unsigned long long;
using vll=vector<ll>;
using vvll = vector<vector<ll>>;
using P = pair<ll,ll>;
using vp=vector<pair<ll, ll>>;

const ll INF=1ll<<60;
ll mod10=1e9+7;
ll mod99=998244353;
const double PI = acos(-1);

#define rep(i,n) for (ll i=0;i<n;++i)
#define per(i,n) for(int i=n-1;i>=0;--i)
#define rep2(i,a,n) for (ll i=a;i<n;++i)
#define per2(i,a,n) for (ll i=a;i>=n;--i)

int main(){
    ll N;
    cin>>N;
    cout << N/2 << endl;
    vector<tuple<ll,ll,ll>> A(N);
    rep(i,N){
    	ll a,b;
    	cin>>a>>b;
    	A[i]={a,b,i};
    }
    sort(all(A));
    rep(i,N/2){
    	ll a,b,c;
    	tie(a,b,c)=A[2*i];
    	ll d;
    	tie(a,b,d)=A[2*i+1];
    	cout<<c+1<<" "<<d+1<<endl;
    }
}