import java.util.*
fun main(args: Array<String>){
	val cin = Scanner(System.`in`)
    val n = cin.nextInt()

    if (n == 3) {
        println(2)
        println("1 3")
        println("2 3")
    } else if (n == 4) {
        println(4)
        println("1 2")
        println("1 3")
        println("4 2")
        println("4 3")
    } else if (n % 2 == 1) {
        println((n/2 + 1)*4 - 4)

        for (i in 0..n/2) {
            if (i == 0) {
                println("$n 1")
                println("$n ${n-1}")
            } else if (i == n/2) {
                println("${n/2} $n")
                println("${n/2+1} $n")
            } else {
                println("$i ${i+1}")
                println("$i ${n-i-1}")
                println("${n-i} ${i+1}")
                println("${n-i} ${n-i-1}")
            }
        }
    } else {
        println(2*n)
        for (i in 0..n/2 - 1) {
            if (i == n/2 - 1) {
                println("1 ${n/2}")
                println("1 ${n/2 + 1}")
                println("$n ${n/2}")
                println("$n ${n/2 + 1}")
            } else {
                println("${i+1} ${i+2}")
                println("${i+1} ${n-i-1}")
                println("${n-i} ${i+2}")
                println("${n-i} ${n-i-1}")
            }
        }
    }
}