modifier inActivePeriod() {
        require((startFirstStepICO < now && now <= endFirstStepICO) || (starSecondStepICO < now && now <= endSecondStepICO));
        _;
    }

    // low level token purchase function
    function buyTokens(address _sender) inActivePeriod payable {
        require(_sender != 0x0);
        require(msg.value >= minTransactionAmount);

        uint256 weiAmount = msg.value;
        raisedForEther = raisedForEther.add(weiAmount);

        // calculate token amount to be created
        uint256 tokens = weiAmount.mul(rate);
        tokens += getBonus(tokens);

        tokenReserve(_sender, tokens);

        forwardFunds();
    }