#include <stdio.h>

void main(){
	int total = 0;
	int inputTotal = scanf("%d",&total);
	if(inputTotal == 2){
		printf("totalの入力ミス\n");
	}
	int eat = 0;
	int inputEat = scanf("%d",&eat);
	if(inputEat == 2){
		printf("eatの入力ミス\n");
	}
	int count = 0;
	while(total > eat*2){
        count++;
		total -= eat*2;
	}
    printf("%d\n",count*eat);
}