import std.conv, std.functional, std.range, std.stdio, std.string;
import std.algorithm, std.array, std.bigint, std.bitmanip, std.complex, std.container, std.math, std.mathspecial, std.numeric, std.regex, std.typecons;
import core.bitop;

class EOFException : Throwable { this() { super("EOF"); } }
string[] tokens;
string readToken() { for (; tokens.empty; ) { if (stdin.eof) { throw new EOFException; } tokens = readln.split; } auto token = tokens.front; tokens.popFront; return token; }
int readInt() { return readToken.to!int; }
long readLong() { return readToken.to!long; }

bool chmin(T)(ref T t, in T f) { if (t > f) { t = f; return true; } else { return false; } }
bool chmax(T)(ref T t, in T f) { if (t < f) { t = f; return true; } else { return false; } }

int binarySearch(alias pred, T)(in T[] as) { int lo = -1, hi = cast(int)(as.length); for (; lo + 1 < hi; ) { const mid = (lo + hi) >> 1; (unaryFun!pred(as[mid]) ? hi : lo) = mid; } return hi; }
int lowerBound(T)(in T[] as, T val) { return as.binarySearch!(a => (a >= val)); }
int upperBound(T)(in T[] as, T val) { return as.binarySearch!(a => (a > val)); }


alias Pt = Tuple!(int, "x", int, "y");

void main() {
  try {
    for (; ; ) {
      const N = readInt;
      auto P = new Pt[N];
      foreach (u; 0 .. N) {
        P[u].x = readInt;
        P[u].y = readInt;
      }
      
      auto us = iota(N).array;
      us.sort!((u, v) => (P[u] < P[v]));
      writeln(N / 2);
      foreach (i; 0 .. N / 2) {
        writeln(us[2 * i] + 1, " ", us[2 * i + 1] + 1);
      }
    }
  } catch (EOFException e) {
  }
}