function getBonus(uint256 _tokens) constant returns (uint256 bonus) {
        require(_tokens != 0);
        if (1 == getCurrentPeriod()) {
            if (startFirstStepICO <= now && now < startFirstStepICO + 1 days) {
                return _tokens.mul(0.1);
            } else if (startFirstStepICO + 1 days <= now && now < startFirstStepICO + 2 days ) {
                return _tokens.mul(0.05);
            } else if (startFirstStepICO + 2 days <= now && now < startFirstStepICO + 3 days ) {
                return _tokens.mul(0.03);
            }
        }

        return 0;
    }