package main

import (
	"fmt"
)

func isPrime(n int) bool {
	if n < 2 {
		return false
	}
	for i := 2; i*i <= n; i++ {
		if n%i == 0 {
			return false
		}
	}
	return true
}

func main() {
	var n int
	if _, err := fmt.Scan(&n); err != nil {
		return
	}

	p := n
	for !isPrime(p) {
		p++
	}

	fmt.Println(p)
	for i := 1; i < n; i++ {
		fmt.Printf("%d %d\n", i, i+1)
	}
	fmt.Printf("%d %d\n", n, 1)

	k := p - n
	half := n / 2
	for i := 1; i <= k; i++ {
		fmt.Printf("%d %d\n", i, i+half)
	}
}