#include <string.h>

#include <algorithm>
#include <array>
#include <bitset>
#include <cassert>
#include <cfloat>
#include <chrono>
#include <ciso646>
#include <climits>
#include <cmath>
#include <complex>
#include <cstdio>
#include <functional>
#include <iomanip>
#include <iostream>
#include <map>
#include <numeric>
#include <optional>
#include <queue>
#include <random>
#include <set>
#include <stack>
#include <string>
#include <unordered_map>
#include <unordered_set>
#include <utility>
#include <vector>

#define REP_OVERLOAD(arg1, arg2, arg3, arg4, NAME, ...) NAME
#define REP3(i, l, r, s)                                               \
    for (int i = int(l), rep3_r = int(r), rep3_s = int(s); i < rep3_r; \
         i += rep3_s)
#define REP2(i, l, r) REP3(i, l, r, 1)
#define REP1(i, n) REP2(i, 0, n)
#define rep(...) REP_OVERLOAD(__VA_ARGS__, REP3, REP2, REP1, )(__VA_ARGS__)
#define repin(i, l, r) for (int i = int(l), repin_r = int(r); i <= repin_r; ++i)

#define RREP_OVERLOAD(arg1, arg2, arg3, arg4, NAME, ...) NAME
#define RREP3(i, l, r, s)                                                      \
    for (int i = int(r) - 1, rrep3_l = int(l), rrep3_s = int(s); i >= rrep3_l; \
         i -= rrep3_s)
#define RREP2(i, l, r) RREP3(i, l, r, 1)
#define RREP1(i, n) RREP2(i, 0, n)
#define rrep(...) RREP_OVERLOAD(__VA_ARGS__, RREP3, RREP2, RREP1, )(__VA_ARGS__)
#define rrepin(i, l, r) \
    for (int i = int(r), rrepin_l = int(l); i >= rrepin_l; --i)

#include <atcoder/modint>

#include <algorithm>
#include <cassert>
#include <vector>

namespace rklib {

// a <- max(a, b)
template <class T>
bool chmax(T &a, const T &b) {
    if (a < b) {
        a = b;
        return true;
    }
    return false;
}

// a <- min(a, b)
template <class T>
bool chmin(T &a, const T &b) {
    if (a > b) {
        a = b;
        return true;
    }
    return false;
}

// if a < 0: a <- b
// else: a <- min(a, b)
template <class T>
bool chmin_non_negative(T &a, const T &b) {
    if (a < 0 || a > b) {
        a = b;
        return true;
    }
    return false;
}

// floor(num / den)
template <class T>
T div_floor(T num, T den) {
    if (den < 0) num = -num, den = -den;
    return num >= 0 ? num / den : (num + 1) / den - 1;
}

// ceil(num / den)
template <class T>
T div_ceil(T num, T den) {
    if (den < 0) num = -num, den = -den;
    return num <= 0 ? num / den : (num - 1) / den + 1;
}

namespace internal {

template <class T>
T remainder_count(T r, T b, T m) {
    return r / m * b + std::min(b, r % m);
}

}  // namespace internal

// Number of integer x s.t.
// - x in [l, r)
// - x mod m in [a, b)
template <class T>
T remainder_count(T l, T r, T a, T b, T m) {
    assert(l >= 0);
    assert(m >= 1);

    if (l >= r || a >= b) return 0;
    if (m <= a || b < 0) return 0;
    chmax(a, T(0));
    chmin(b, m);

    auto res = internal::remainder_count(r, b, m);
    if (l >= 1) res -= internal::remainder_count(l, b, m);
    if (a >= 1) res -= internal::remainder_count(r, a, m);
    if (l >= 1 && a >= 1) res += internal::remainder_count(l, a, m);

    return res;
}

}  // namespace rklib


using namespace std;
using namespace rklib;

using lint = long long;
using pii = pair<int, int>;
using pll = pair<lint, lint>;

int main() {
    int n;
    cin >> n;
    vector<int> x(n), y(n);
    rep(i, n) cin >> x[i] >> y[i];

    vector<pii> ys[8010];  // (y, idx)
    rep(i, n) ys[x[i]].push_back({y[i], i});

    {
        int cur = 0;
        rep(i, n) if (!ys[i].empty()) {
            if (cur % 2 == 0) {
                sort(ys[i].begin(), ys[i].end());
            } else {
                sort(ys[i].begin(), ys[i].end(), greater<>());
            }
            ++cur;
        }
    }
    vector<int> vs;
    rep(i, 8010) for (auto [y, idx] : ys[i]) vs.push_back(idx + 1);
    cout << vs.size() / 2 << "\n";
    rep(i, 0, vs.size(), 2) { cout << vs[i] << " " << vs[i + 1] << "\n"; }
}