#include <bits/extc++.h>
#include <atcoder/all>

using namespace std;
using ll = long long;
#define REP(i,n) for(int i=0;i<int(n);i++)
#define FOR(i,a,b) for(int i=a;i<=int(b);i++)
#define ALL(x) x.begin(),x.end()
#define INF INT_MAX
#define INFL LLONG_MAX / 4
using namespace atcoder;
using mint = modint998244353;
template<typename T> void chmin(T& a, T b) { a = min(a, b); }
template<typename T> void chmax(T& a, T b) { a = max(a, b); }
#define PR(x) cerr << #x << "=" << x << endl
using i128 = __int128_t;

int main() {
  int n;
  cin >> n;
  vector<tuple<int, int, int>> xy(n);
  REP(i, n) {
    int x, y;
    cin >> x >> y;
    xy[i] = {x,y,i+1};
  }

  sort(ALL(xy));

  cout << n / 2 << endl;
  REP(i, n / 2) {
    cout << get<2>(xy[2*i]) << ' ' << get<2>(xy[2*i+1]) << endl;
  }


  return 0;
}