def main():
	l,k = map(int,input().split())
	length = l
	eat = 0
	'''while(length > k*2):
		eat += k
		length -= k*2
	print(eat)''' #TLEになる

	count = length // (k*2)

	if(length % (k*2) != 0):
		print(count*k)
	else:
		print(((count-1)*k))

if __name__ == '__main__':
	main()