import sys

def check_range(a,b,x):
	if a > x or b < x:
		sys.exit()

def check_eq(a,b):
	if a != b:
		sys.exit()

data = input().split(' ')

L = int(data[0])
check_range(1,1000000000,L)

K = int(data[1])
check_range(1,50,K)

#data = input().split(' ')
#check_eq(len(data),N)

#for i in range(0,N):
#	data[i] = int(data[i])
#	check_range(1,L,data[i])

if L % (2 * K) == 0:
	yu = L // (2 * K) - 1
else:
	yu = L // (2 * K)

print(yu * K)