function default_helper() payable {
    if (bought_tokens) throw;
    
    uint256 deposit = msg.value;
    if (this.balance > min_required_amount) {
      uint256 refund = this.balance - min_required_amount;
      deposit -= refund;
      msg.sender.transfer(refund);
    }
    balances[msg.sender] += deposit;
  }