import math

n = list(map(int, input().split()))

if n[1] * 2 >= n[0]:
    print('0')
elif n[0] % n[1] == 0:
    print(int(((n[0] / (n[1] * 2)) - 1) * n[1]))
else:
    number = math.floor(n[0] / (n[1] * 2))
    print(number * n[1])