#include <bits/stdc++.h>
using namespace std;
#define overload2(a, b, c, ...) c
#define overload3(a, b, c, d, ...) d
#define overload4(a, b, c, d, e ...) e
#define overload5(a, b, c, d, e, f ...) f
#define overload6(a, b, c, d, e, f, g ...) g
#define fast_io ios::sync_with_stdio(false); cin.tie(nullptr);
#pragma GCC optimize("Ofast,no-stack-protector,unroll-loops,fast-math")
typedef long long ll;
typedef long double ld;
#define chmin(a,b) a = min(a,b);
#define chmax(a,b) a = max(a,b);
#define bit_count(x) __builtin_popcountll(x)
#define leading_zero_count(x) __builtin_clz(x)
#define trailing_zero_count(x) __builtin_ctz(x)
#define gcd(a,b) __gcd(a,b)
#define lcm(a,b) a / gcd(a,b) * b
#define rep(...) overload3(__VA_ARGS__, rrep, rep1)(__VA_ARGS__)
#define rep1(i,n) for(int i = 0 ; i < n ; i++)
#define rrep(i,a,b) for(int i = a ; i < b ; i++)
#define repi(it,S) for(auto it = S.begin() ; it != S.end() ; it++)
#define pt(a) cout << a << endl;
#define print(...) printall(__VA_ARGS__);
#define debug(a) cout << #a << " " << a << endl;
#define all(a) a.begin(), a.end()
#define endl "\n";
#define v1(T,n,a) vector<T>(n,a)
#define v2(T,n,m,a) vector<vector<T>>(n,v1(T,m,a))
#define v3(T,n,m,k,a) vector<vector<vector<T>>>(n,v2(T,m,k,a))
#define v4(T,n,m,k,l,a) vector<vector<vector<vector<T>>>>(n,v3(T,m,k,l,a))
template<typename T,typename U>istream &operator>>(istream&is,pair<T,U>&p){is>>p.first>>p.second;return is;}
template<typename T,typename U>ostream &operator<<(ostream&os,const pair<T,U>&p){os<<p.first<<" "<<p.second;return os;}
template<typename T>istream &operator>>(istream&is,vector<T>&v){for(T &in:v){is>>in;}return is;}
template<typename T>ostream &operator<<(ostream&os,const vector<T>&v){for(auto it=v.begin();it!=v.end();){os<<*it<<((++it)!=v.end()?" ":"");}return os;}
template<typename T>istream &operator>>(istream&is,vector<vector<T>>&v){for(T &in:v){is>>in;}return is;}
template<typename T>ostream &operator<<(ostream&os,const vector<vector<T>>&v){for(auto it=v.begin();it!=v.end();){os<<*it<<((++it)!=v.end()?"\n":"");}return os;}
template<typename T>ostream &operator<<(ostream&os,const set<T>&v){for(auto it=v.begin();it!=v.end();){os<<*it<<((++it)!=v.end()?" ":"");}return os;}
template<typename T>ostream &operator<<(ostream&os,const multiset<T>&v){for(auto it=v.begin();it!=v.end();){os<<*it<<((++it)!=v.end()?" ":"");}return os;}
template<class... Args> void printall(Args... args){for(auto i:initializer_list<common_type_t<Args...>>{args...}) cout<<i<<" ";cout<<endl;}


void solve(){
    int n;
    cin >> n;
    vector<tuple<int,int,int,int>> V(n);
    rep(i,n){
        int a, b;
        cin >> a >> b;
        V[i] = {a+b,a,b,i};
    }
    sort(all(V));
    pt(n/2)
    for(int i = 1; i < n; i+=2){
        auto[nd,nx,ny,nid] = V[i];
        auto[d,x,y,id] = V[i-1];
        print(id+1,nid+1)
    }
}

int main(){
    fast_io
    int t = 1;
    // cin >> t;
    rep(i,t) solve();
}