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])

haruka = 0
yu = 0	

while L - 2 * K > 0:
	haruka += K
	yu += K
	L -= 2 * K

print(yu)