```go
package main

import "fmt"

func main() {
	var n int
	fmt.Scan(&n)

	k := n / 2
	m := k * (n - k)
	fmt.Println(m)

	for i := 1; i <= k; i++ {
		for j := k + 1; j <= n; j++ {
			fmt.Println(i, j)
		}
	}
}
```