#include <stdio.h>

int main(void)
{
	long l,ans;
	char k;
	scanf("%ld %hhd",&l,&k);

	ans = 0;
	l -= 2*k;
	while (l>0) {
		l -= 2*k;
		ans += k;
	}

	printf("%ld\n",ans);
	return 0;
}