```go
package main

import (
	"bufio"
	"fmt"
	"os"
)

type Pair struct {
	a, b int
}

func main() {
	in := bufio.NewReaderSize(os.Stdin, 1<<20)
	var n int
	fmt.Fscan(in, &n)

	comp := make([]bool, n+1)
	primes := make([]int, 0)
	for i := 2; i <= n; i++ {
		if !comp[i] {
			primes = append(primes, i)
			if i*i <= n {
				for j := i * i; j <= n; j += i {
					comp[j] = true
				}
			}
		}
	}

	used := make([]bool, n+1)
	pairs := make([]Pair, 0, n/2)

	for idx := len(primes) - 1; idx >= 0; idx-- {
		p := primes[idx]
		if p == 2 {
			continue
		}
		v := make([]int, 0)
		for x := p; x <= n; x += p {
			if !used[x] {
				v = append(v, x)
			}
		}
		if len(v)%2 == 1 {
			if 2*p > n {
				continue
			}
			u := make([]int, 0, len(v)-1)
			removed := false
			skip := 2 * p
			for _, x := range v {
				if !removed && x == skip {
					removed = true
					continue
				}
				u = append(u, x)
			}
			v = u
		}
		for i := 0; i+1 < len(v); i += 2 {
			a, b := v[i], v[i+1]
			used[a] = true
			used[b] = true
			pairs = append(pairs, Pair{a, b})
		}
	}

	evens := make([]int, 0)
	for x := 2; x <= n; x += 2 {
		if !used[x] {
			evens = append(evens, x)
		}
	}
	for i := 0; i+1 < len(evens); i += 2 {
		pairs = append(pairs, Pair{evens[i], evens[i+1]})
	}

	out := bufio.NewWriterSize(os.Stdout, 1<<20)
	fmt.Fprintln(out, len(pairs))
	for _, p := range pairs {
		fmt.Fprintln(out, p.a, p.b)
	}
	out.Flush()
}
```