package main

import (
	"fmt"
	"strings"
)

func main() {
	var l int
	_, _ = fmt.Scan(&l)
	s := "a" + strings.Repeat("ba", (l-1)/2)

	cnt := 0
	for i := 0; i < l-2; i++ {
		t := s[i+1:]
		cnt += len(t) / 2
	}
	fmt.Println(cnt)
}