package main

import "fmt"

func scan() (L int, K int) {
	fmt.Scan(&L, &K)
	return L, K
}

func divmod(numerator, denominator int) (quotient, remainder int) {
	quotient = numerator / denominator
	remainder = numerator % denominator
	return

}

func main() {
	L, K := scan()
	// L, K := 90, 20
	// L, K := 100, 50
	q, r := divmod(L, K*2)

	if r == 0 {
		fmt.Print((q - 1) * K)
	} else {
		fmt.Print(q * K)
	}
}