package main

import . "fmt"

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