package main

import . "fmt"
import . "sort"

func main() {
	var a,b int
	Scan(&a,&b)
	a+=(b-a%b)%b
	c:=Search(a/b+1,func(i int)bool{
	 return i+i>a/b
	 
	})
	Println(a-c*b)
}