#include <stdio.h>

int main() {

	int pocky, haru, a = 0;

	scanf("%d%d", &pocky, &haru);

	while (pocky > haru) {
		pocky -= 2 * haru;
		a++;
	}
	if (pocky <= 0) {
		a -= 1;
		printf("%d\n", haru * a);
	}
	else {
		printf("%d\n", haru * a);
	}
	return 0;
}