#include <bits/stdc++.h>

#include <atcoder/all>
using ll = long long;
#define rep(i, n) for (int i = 0, i##_len = (n); i < i##_len; ++i)
#define reps(i, n) for (int i = 1, i##_len = (n); i <= i##_len; ++i)
#define rrep(i, n) for (int i = ((int)(n)-1); i >= 0; --i)
#define rreps(i, n) for (int i = ((int)(n)); i > 0; --i)
#define rep2(i, s, n) for (int i = (s); i < (int)(n); ++i)
#define repc2(i, s, n) for (int i = (s); i <= (int)(n); ++i)
constexpr int inf = 2000'000'000;
constexpr ll linf = 4000000000000000000ll;
constexpr ll M7 = 1000000007ll;
constexpr ll M09 = 1000000009ll;
constexpr ll M9 = 998244353ll;
#define all(v) begin(v), end(v)
#define rall(v) rbegin(v), rend(v)
using namespace std;
using namespace atcoder;
template <typename T>
inline ostream& operator<<(ostream& os, vector<T>& v) {
    for (auto& e : v) os << e << " ";
    return os;
}
template <typename T, typename U>
std::ostream& operator<<(std::ostream& os, const std::pair<T, U>& p) noexcept {
    return os << "(" << p.first << ", " << p.second << ")";
}
#ifdef ONLINE_JUDGE
#define debug(...)
#else
#define debug(...) cerr << "<" << #__VA_ARGS__ << ">: ", debug_out(__VA_ARGS__)
template <typename T>
void debug_out(T t) {
    cerr << t << endl;
}
template <typename T, typename... Args>
void debug_out(T t, Args... args) {
    cerr << t << ", ";
    debug_out(args...);
}
#endif

int main() {
    ios_base::sync_with_stdio(false);
    cin.tie(NULL);
    int n;
    cin >> n;
    using P = pair<int, int>;
    vector<P> v(n);
    rep(i, n) {
        int a, b;
        cin >> a >> b;
        v[i] = {a, b};
    }
    vector<int> idx(n);
    iota(all(idx), 0);
    sort(all(idx), [&](int i, int j) { return v[i] < v[j]; });
    cout << n / 2 << "\n";
    rep(i, n / 2) {
        cout << idx[i * 2] + 1 << " " << idx[i * 2 + 1] + 1 << "\n";
    }
    return 0;
}