function rewardTokenAmount(uint256 buyTokenAmount) public view returns (uint256) {
        if (!isOpen()) {
            return 0;
        }

        uint256 rewardTokenPercent = 0;

        
        uint256 timePeriod = block.timestamp.sub(openingTime()).div(_rewardPeriod);

        if (timePeriod < 1) {
            rewardTokenPercent = 15;
        } else if (timePeriod < 2) {
            rewardTokenPercent = 10;
        } else if (timePeriod < 3) {
            rewardTokenPercent = 5;
        } else {
            return 0;
        }

        return buyTokenAmount.mul(rewardTokenPercent).div(100);
    }