Ethereum 如何声明实体中的常量

我真的是刚接触过稳定和智能合约，非常感谢您的帮助。我遵循一个教程，这是他们使用的确切代码。但是当我编译代码时，我得到了以下错误：

ParserError:应为主表达式。 地址公共常量审批人=

pragma solidity ^0.6.0;

contract ApprovalContract {

    address public sender;
    address public receiver;
    address public constant approver = ;

    function deposit(address _receiver) external payable {
        require(msg.value > 0);
        sender = msg.sender;
        receiver = _receiver;
    }

    function viewApprover() external pure returns(address) {
        return(approver);
    }

    function approve() external {
        require(msg.sender == approver);
        receiver.transfer(address(this).balance);
    }
}

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该常量需要初始化

address public constant approver = YOURADDRESS;