#include <stdio.h>

int main (void){
	int l,k,h,y;
	scanf("%d%d",&l,&k);
	y=0; h=l;
	for(;;){
		if(y<h && h-y >2*k){
			y += k;
			h -= k;
		}else break;
	}
	printf("%d",y);
	return(0);
}