#include <bits/stdc++.h> 
using namespace std;
using ll =long long;
#define all(v) v.begin(),v.end()
 #define rep(i,a,b) for(int i=a;i<b;i++)
#define rrep(i,a,b) for(int i=a;i>=b;i--)

ll INF=2e18;


int main() {
    ios::sync_with_stdio(false);
  cin.tie(0);
ll N;cin>>N;
vector<ll> x(N),y(N);
for(ll i=0;i<N;i++) cin>>x[i]>>y[i];


priority_queue<tuple<ll,ll,ll>,vector<tuple<ll,ll,ll>>,greater<tuple<ll,ll,ll>>> S;

for(ll i=0;i<N;i++) {
  S.push(make_tuple(x[i],y[i],i));
}

vector<pair<ll,ll>> ans(0);
while(S.size()>1) {
auto p=S.top();
S.pop();
auto q=S.top();
S.pop();
ans.push_back(make_pair(get<2>(p),get<2>(q)));
}

cout<<(ll)ans.size()<<endl;
for(auto x:ans) cout<<x.first+1<<" "<<x.second+1<<endl;
}