#include<stdio.h>

int main() {
	int pocky = 0;
	int mouse = 0;

	// 値取得
	scanf("%d %d", &pocky, &mouse);

	// 結果表示
	if (pocky % mouse == 0 && pocky / mouse % 2 == 0)
		printf("%d", pocky / mouse / 2 * mouse - mouse);
	else
		printf("%d", pocky / mouse / 2 * mouse);

	return 0;
}