#include <stdio.h>

int main(void)
{
	unsigned long l, k;
	unsigned long yu = 0;

	do {
		scanf("%lu", &l);
	} while (l < 1 || l > 1000000000);

	do {
		scanf("%lu", &k);
	} while (k < 1 || k > 50);

	while (1) {
		if (l - 2 * yu <= 2 * k)
			break;
		yu += k;
	}

	printf("%lu\n", yu);

	return 0;
}