import math

l, b = input().split()

L = int(l)
B = int(b)

if L / (2 * B) <= 1:
    count = 0
elif L / (2 * B) >1:
    if L % (2 * B) == 0:
        count = L/(2*B) - 1
    else:
        count = math.floor(L/(2*B)) 

print(int(count * B))