# coding:utf-8

L, K = map(int,input().split())

haruka_K = 0
yuu_K = 0

K_length = L // (2 * K)
nokori_L = L % (2 * K)

if L <= 2 * K:
    print(0)
else:
    haruka_K = (L // 2) + nokori_L

print(L - haruka_K)