function complete_buy_exchange() private {
    
    uint amount_give_ = msg.value;

    uint amount_get_ = get_amount_get_buy(amount_give_);
    
    uint amount_get_minus_fee_ = (amount_get_.mul(1 ether - commission_ratio)).div(1 ether);

    uint admin_fee = amount_get_ - amount_get_minus_fee_;

    transferTokens(msg.sender, amount_get_minus_fee_);
    
    transferETH(admin, admin_fee);
    
  }