why add a negative sign before value in the function backup of the class Node ,I can‘t understand ?

[cheng-lao]

#这是你的源代码
def __update(self, value: float):
        """ 更新节点的访问次数 `N(s, a)`、节点的累计平均奖赏 `Q(s, a)`

        Parameters
        ----------
        value: float
            用来更新节点内部数据
            
        """
        self.Q = (self.N * self.Q + value)/(self.N + 1)
        self.N += 1

    def backup(self, value: float):
        """ 反向传播 """
        if self.parent:
            self.parent.backup(-value)  #there is the negative sign, why ?

        self.__update(value)

thanks advance ！