#include #ifdef LOCAL #include #else #define debug(...) void(0) #endif using namespace std; typedef long long ll; #define all(x) begin(x), end(x) constexpr int INF = (1 << 30) - 1; constexpr long long IINF = (1LL << 60) - 1; constexpr int dx[4] = {1, 0, -1, 0}, dy[4] = {0, 1, 0, -1}; template istream& operator>>(istream& is, vector& v) { for (auto& x : v) is >> x; return is; } template ostream& operator<<(ostream& os, const vector& v) { auto sep = ""; for (const auto& x : v) os << exchange(sep, " ") << x; return os; } template bool chmin(T& x, U&& y) { return y < x and (x = forward(y), true); } template bool chmax(T& x, U&& y) { return x < y and (x = forward(y), true); } template void mkuni(vector& v) { sort(begin(v), end(v)); v.erase(unique(begin(v), end(v)), end(v)); } template int lwb(const vector& v, const T& x) { return lower_bound(begin(v), end(v), x) - begin(v); } const int MAX = 8010; int main() { ios::sync_with_stdio(false); cin.tie(nullptr); int N; cin >> N; vector>> ps(MAX); for (int i = 0; i < N; i++) { int x, y; cin >> x >> y; ps[x].emplace_back(y, i); } vector> ans; for (int i = 0, pre = -1; i < MAX; i++) { sort(all(ps[i])); for (auto [val, idx] : ps[i]) { if (pre == -1) pre = idx; else { ans.emplace_back(pre, idx); pre = -1; } } } cout << ans.size() << '\n'; for (auto [a, b] : ans) cout << a + 1 << ' ' << b + 1 << '\n'; return 0; }