L, K= map(int, input().split())
count= L / (K*2)
T_count= int(count)
term= L % (K*2)

if term != 0:
	if count <= 1:
		U_length= 0
	else:
		U_length= K * T_count
else:
	U_length= K * T_count - K
	
print(U_length)