package main

import . "fmt"

func main() {
	var a,b,c int
	Scan(&a,&b)
	if a%b==0 {
		c=a/b
		if c%2==0 {
			c=c/2-1
		} else {
			c/=2
		}
		
	} else {
		c=a/2/b
	}
	Println(c*b)
}