package main

import (
	"fmt"
)

func resolve(L, K int) int {
	if L%(2*K) == 0 {
		L -= 1
	}
	return K * (L / (2 * K))
}

func main() {
	var L, K int
	fmt.Scanf("%d %d\n", &L, &K)
	fmt.Println(resolve(L, K))
}