class Problem0063:
	def solve(this):
		l,k = map(int, input().split())
		
		res = l // (k * 2)
		
		cnt = 0
		if l % (k * 2) == 0:
			cnt = res - 1
		else:
			cnt = res
		print(cnt * k)
	
			
if __name__ == "__main__":
	problem = Problem0063()
	problem.solve()