function changeTimes(uint256 _presaleOpeningTime, uint256 _presaleClosingTime, uint256 _openingTime, uint256 _closingTime) public onlyOwner {
        require(!isFinalized);
        require(_presaleOpeningTime >= now);
        require(_presaleClosingTime >= _presaleOpeningTime);
        require(_openingTime >= _presaleClosingTime);
        require(_closingTime >= _openingTime);

        presaleOpeningTime = _presaleOpeningTime;
        presaleClosingTime = _presaleClosingTime;
        openingTime = _openingTime;
        closingTime = _closingTime;

        emit CrowdsaleTimesChanged(_presaleOpeningTime, _presaleClosingTime, _openingTime, _closingTime);
    }