    function notifyRewardAmount(uint256 _amount) external onlyOwner updateReward(address(0)) {
        if (block.timestamp >= finishAt) {
            rewardRatio = _amount / duration;
        } else {
            uint256 remainingRewards = (finishAt - block.timestamp) * rewardRatio;
            rewardRatio = (_amount + remainingRewards) / duration;
        }

        require(rewardRatio > 0, "reward ratio = 0");

        finishAt = block.timestamp + duration;
        updatedAt = block.timestamp;
        emit NotifyRewardChanged(_amount, block.timestamp);
    }

    function _min(uint256 x, uint256 y) private pure returns (uint256) {
        return x <= y ? x : y;
    }
}
