import java.io.*;
import java.util.*;
import java.util.stream.*;

public class Main {
    static final int MOD = 998244353;
    public static void main(String[] args) throws Exception {
        Scanner sc = new Scanner();
        int n = sc.nextInt();
        Point[] points = new Point[n];
        for (int i = 0; i < n; i++) {
            points[i] = new Point(i + 1, sc.nextInt(), sc.nextInt());
        }
        Arrays.sort(points);
        System.out.println(n / 2);
        StringBuilder sb = new StringBuilder();
        for (int i = 0; i < n / 2; i++) {
            sb.append(points[i * 2].idx).append(" ").append(points[i * 2 + 1].idx).append("\n");
        }
        System.out.print(sb);
    }
    
    static class Point implements Comparable<Point> {
        int idx;
        int x;
        int y;
        
        public Point(int idx, int x, int y) {
            this.idx = idx;
            this.x = x;
            this.y = y;
        }
        
        public int compareTo(Point another) {
            if (x == another.x) {
                return y - another.y;
            } else {
                return x - another.x;
            }
        }
    }
}
class Utilities {
    static String arrayToLineString(Object[] arr) {
        return Arrays.stream(arr).map(x -> x.toString()).collect(Collectors.joining("\n"));
    }
    
    static String arrayToLineString(int[] arr) {
        return String.join("\n", Arrays.stream(arr).mapToObj(String::valueOf).toArray(String[]::new));
    }
}
class Scanner {
    BufferedReader br = new BufferedReader(new InputStreamReader(System.in));
    StringTokenizer st = new StringTokenizer("");
    StringBuilder sb = new StringBuilder();
    
    public Scanner() throws Exception {
        
    }
    
    public int nextInt() throws Exception {
        return Integer.parseInt(next());
    }
    
    public long nextLong() throws Exception {
        return Long.parseLong(next());
    }
    
    public double nextDouble() throws Exception {
        return Double.parseDouble(next());
    }
    
    public int[] nextIntArray() throws Exception {
        return Stream.of(br.readLine().split(" ")).mapToInt(Integer::parseInt).toArray();
    }
    
    public String next() throws Exception {
        while (!st.hasMoreTokens()) {
            st = new StringTokenizer(br.readLine());
        }
        return st.nextToken();
    }
    
}