/*
MMMMMMMMMMMMMMMMMMMMMMMMMMMMMMMMMMMMMMMMMMMMMMMMMMMMMMMMMMMMMMMMMMMMMMMMMMMMMMMMMMMMMMMMMMMMMMMMMMMMMMMMMMMMMMMMMMMMMMMMMMMMMMMMMMMMMMMMMMMMMMMMMMMMMM
MMMMMMMMMMMMMMMMMMMMMMMMMMMMMMMMMMMMMMMMMMMMMMMMMMMMMMMMMMMWNXK0OxdoollccccclodkOKNWMMMMMMMMMMMMMMMMMMMMMMMMMMMMMMMMMMMMMMMMMMMMMMMMMMMMMMMMMMMMMMMMMM
MMMMMMMMMMMMMMMMMMMMMMMMMMMMMMMMMMMMMMMMMMMMMMMMMMMMWX0kdlc;'..      .,:loxkk0KXNWWMMMMMMMMMMMMMMMMMMMMMMMMMMMMMMMMMMMMMMMMMMMMMMMMMMMMMMMMMMMMMMMMMMM
MMMMMMMMMMMMMMMMMMMMMMMMMMMMMMMMMMMMMMMMMMMMMMMNKkoc,..        .':ox0XNWMMMMMMMMMMMMMMMMMMMMMMMMMMMMMMMMMMMMMMMMMMMMMMMMMMMMMMMMMMMMMMMMMMMMMMMMMMMMMM
MMMMMMMMMMMMMMMMMMMMMMMMMMMMMMMMMMMMMMMMMMWXOd:'.          .;lxKNWMMMMMMMMMMMMMMMWWNNNNNNNNWWWWMMMMMMMMMMMMMMMMMMMMMMMMMMMMMMMMMMMMMMMMMMMMMMMMMMMMMMM
MMMMMMMMMMMMMMMMMMMMMMMMMMMMMMMMMMMMMMMNOo:.           .,lkXWMMMMMMMMMWXKOxddol:;;,''.....'',,;:cldxO0XWMMMMMMMMMMMMMMMMMMMMMMMMMMMMMMMMMMMMMMMMMMMMMM
MMMMMMMMMMMMMMMMMMMMMMMMMMMMMMMMMMMWXxc'            .;d0NMMMMMMMWXOxl:,..                            ..,:ldOKNMMMMMMMMMMMMMMMMMMMMMMMMMMMMMMMMMMMMMMMM
MMMMMMMMMMMMMMMMMMMMMMMMMMMMMMMMWXx:.            .;dKWMMMMMWN0dc,.                                          .,cdOXWMMMMMMMMMMMMMMMMMMMMMMMMMMMMMMMMMMM
MMMMMMMMMMMMMMMMMMMMMMMMMMMMMMNkc.             'l0NMMMMMN0d:'.                                                   .:d0NMMMMMMMMMMMMMMMMMMMMMMMMMMMMMMMM
MMMMMMMMMMMMMMMMMMMMMMMMMMMW0o'             .;xXMMMMMNOo,.              .....''',,'''....                           .'lkXWMMMMMMMMMMMMMMMMMMMMMMMMMMMM
MMMMMMMMMMMMMMMMMMMMMMMMMNOc.             .:kNMMMMW0o,         ..,:loxk0KKXXNNNWWWNNNNXKK0kxol:,..                      .cONMMMMMMMMMMMMMMMMMMMMMMMMMM
MMMMMMMMMMMMMMMMMMMMMMMNk;.             .;ONMMMMXx;.      .,cdkKNWMMMMMMMMMMMMMMMMMMMMMMMMMMMMMWNKOdc;.                    ,oKWMMMMMMMMMMMMMMMMMMMMMMM
MMMMMMMMMMMMMMMMMMMMMNk;               ,kNMMMW0l.     ':dOXWMMMMMMMMMMMMMMMMMMMMMMMMMMMMMMMMMMMMMMMMMWXOd:'                  .cOWMMMMMMMMMMMMMMMMMMMMM
MMMMMMMMMMMMMMMMMMMWO:               .dXMMMW0c.   .;oONWMMMMMMMMMMMMMMMMMMMMMMMMMMMMMMMMMMMMMMMMMMMMMMMMMMNOo;.                .:ONMMMMMMMMMMMMMMMMMMM
MMMMMMMMMMMMMMMMMWKc.               :0WMMWKl.  .;dKWMMMMMMMMMMMMMMMMMMMMMMMMMMMMMMMMMMMMMMMMMMMMMNWMMMMMMMMMMWKx:.               .:OWMMMMMMMMMMMMMMMMM
MMMMMMMMMMMMMMMMNx.               .dNMMMXd.  ,o0WMMMMMMMMMMMMMMMMMMMMMMMMMMMMMMMMMMMMMMMMMMMMMMMMN0kxOXWMMMMMMMMWKd;.              .cKWMMMMMMMMMMMMMMM
MMMMMMMMMMMMMMW0:                ,OWMMWO, .:kNMMMMMMMMMMMMMMMMMMMMMMWWWWMMMMMMMMMMMMMMMMMMMMMMMMMMWXxc,:d0NMMMMMMMMNOc.              .xNMMMMMMMMMMMMMM
MMMMMMMMMMMMMNx.                :KMMMXo..c0WMMMMMMMMMMMMMMMMMMMMXxl:;,;;:codkKNMMMMMMMMMMMMMMMMMMMMMMNk;..,o0WMMMMMMMWKo.              :KMMMMMMMMMMMMM
MMMMMMMMMMMMXl.                cXMMW0:.c0WMMMMMMMMMMMMWWNNXXXNNWXd'..;;'..   .,lkNMMW00NMMMMMMMMMMMMMMMNk;  .;dKWMMMMMMWKo.             'kWMMMMMMMMMMM
MMMMMMMMMMMK:                 cXMMWk;;OWMMMMMMMMMN0xoc;,'.....';cll'.oKNX0xol,   ,xXNc.,xXMMMMMMMMMMMMMMMNx'   .l0WMMMMMMWKl.            .dNMMMMMMMMMM
MMMMMMMMMM0;                 cXMMWk:dXMMMMMMMMW0o,.  ..,:clllllcc:;,..;0WMMMMNO:.  ;Ol.  ;0MMMMMMMMMMMMMMMMXl.   .:OWMMMMMMWO;            .lNMMMMMMMMM
MMMMMMMMM0,                 ;KMMWklkNWMMMMMMMKl.     .';coxOXWMMMMWNKOdxKWMMMMMWk,  .:l:  ,0MMMMMMMMMMMMMMMMWO,    .c0WMMMMMMXo.            lXMMMMMMMM
MMMMMMMM0,                 '0MMNx,''lXMMMMMWk'   .,cdxkkkxdxxOKXWMMMMMMWNXNMMMMMM0,  .kK;  cNMMMMMMN0XMMMMMMMMKc     .oXMMMMMMWO,            lNMMMMMMM
MMMMMMM0;                 .xWMKc. . .kWMMMNd.  .l0NMMMMMMMMMMWNXNWMMMMMMMWNNWMMMMMk. 'OWd. .OMMMMXx:oXMMMMMMMMMNo.     ,kWMMMMMMK:           .dWMMMMMM
MMMMMMX:                  cXM0; .l0o..:ool,  .lKWMMMMMMMMMMMMMMMMMMMMMMMMMMMMMMMMMX; ;XMO. .dWWKo'.:KMMMMMMMMMMMWx.     .oXMMMMMMXc           .kWMMMMM
MMMMMNl                  .OMX: .oNMWOc'...':dKWMMMMMMMMMMMMMMMMMMMMMMMMMMMMMMMMMMMNc.dWMO.  lOl.  ,0MMMMMMMMMMMMMWx.      :KMMMMMMXl           ;KMMMMM
MMMMWx.                  lNMx. ;XMMMMMNXKXNWMWNWMMMMMMMMMMMMMMMXO0WMMMMMMMMMMMMMMMK::XMWx.  ...'.'OMMMMMMMMMMMMMMMWd.      ;0MMMMMMXc           oWMMMM
MMMMK,                  .kMNl  cNMMMMMMMWNX0kxONMMMMMMMMMMMMMMWd..,cok0NWMMMMMMMMM0xKMMX;   .cd;'kWMMMMMMMMMMMMMMMMNo       ,0MMMMMMK:          '0MMMM
MMMWo                   .cl:. .,ooolllllc::ld0WMMMMMMMMMMMMMMMNl      .':oOXNMMMMMWMMWO:  ;d00:.xWMMMMMMMMMMMMMMMMMMX:       ;KMMMMMM0'          oWMMM
MMM0'                    .':ok0KKK0OkxxkOKNWMMMMMMMMMMMMMMMMMMWo          ..;OWMMMMWO:.  ,OWK:.dWMMMMMMMMMMMMMMMMMMMMO'       :XMMMMMWd.         ,KMMM
MMWo                   .l0NWMMMMMMMMMMMMMMMMMMMMMMMMWXKWMMMMW0o'             .kWMWO:. .cOOOO:.oNMMMMMMMMMNXWMMMMMMMMMWo       .oWMMMMMX:         .kMMM
MMK,                   .OMMMMMMMMMMMMMMMMMMMMMMMMN0d:.cXMMMKc.                .ok:. ..;KMMX:.lNMMMMMMMMMWxlKMMMMMMMMMM0,       'OMMMMMWk.         oWMM
MMx.                   'OXXNMMMMMMMMMMMMMMMMMWXkl,.   lWMWO'                       ,O0dd0Kc .kMMMMMMMMMWk' cXMMMMMMMMMWl        lNMMMMMX;         cNMM
MNl                    .dOKNMMMMMMMMMMMMMMWKxc.       lWM0,                        'xXWN0:   ;0MMMWN0xl,.   'cdOXWMMMMMk.       'OMMMMMWd.        ;XMM
MX;                    ;XMMMMMMMMNOkXMMWKx;.          ,KWo                           .:c,     ;KMMWN0ko;.   'cdOXWMMMMMK,        dWMMMMMO.        ,KMM
M0'                    ,KMMMMXKN0:.oWW0c.              ;d;                                     lNMMMMMMWO'.dNMMMMMMMMMMNc        :XMMMMMK,        ;KMM
MO.                    '0MMM0lkO, 'ONd.                                                        .oNMMMMMMWkdNMMMMMMMMMMMWo        ,KMMMMMX:        ;XMM
Mk.                    .OMM0,,x;  ;Ol                                     ..',.                 ;KMMMMMMMNNMMMMMMMMMMMMWo        '0MMMMMNc        cNMM
Mx.                    .dW0, ,:.  ,c.                                      .:0Xk:.              lNMMMMMMMMMMMMMMMMMMMMMWo        .OMMMMMNc        oWMM
Mk.                     c0:  ..    .                                      .,xOd0NO;            '0MMMMMMMMMMMMMWWMMMMMMMWo        .OMMMMMN:       .kMMM
Mk.                     .:.                     .,.                       .cx: .lKXl           lWMMMMMMMMMMMWNxlONMMMMMNc        '0MMMMMX;       ;XMMM
M0'                                           .:c.                      .. .,:;,,l0Xl         .xMMMMMMMMMMMWNk,.:ONMMMMX;        ;XMMMMM0'      .dWMMM
MK;                                          'dl.                        .....''',:k0d;.       oWMMMMMMMMMMMMWOo0MMMMMMO.        oWMMMMMx.      ,KMMMM
MNc                                         :kl.                           ......   ,c;.       :XMMMMMMMMMMMMMWWWMMMMMWo        .kMMMMMNc      .dWMMMM
MWd.                                       c0l.                                                .OMMMMMMMMMMMMMMMMMMMMMK;        :XMMMMMO'      :XMMMMM
MM0'                                      :0d.                                         .        lNMMMMMMMMMMMMMMMMMMMWd.       .kMMMMMNl      '0MMMMMM
MMNl                                     '0k.                    ..                    'c.      .OMMMMMMMMMMMMMMMMMMM0,        lNMMMMMO.     .kWMMMMMM
MMMO.                                   .dK;                    ;0O;                    :d'      ;KMMMMMMMMMMMMMMMMMNl        ,KMMMMMX:     .xWMMMWXNM
MMMNc                                   ,0d.                   .OMMNk:.                 .x0xo,    cXMMMMMMMMMMMMMMMWd.       'OMMMMMNo     .dWMMMWOkNM
MMMMO'                                  c0;                   .dWMMMMW0l.                ;dooxl.   :0WMMMMMMMMMMMMWk.       .kWMMMMWx.    .xWMMMWxc0MM
MMMMWd.                                 ok.                   :XMMMMMMMWO,                  .:dc    'OWMMMMMMMMMMWO'       .kWMMMMWk.    'kWMMMNo,dWMM
MMMMMX:                                 cd.                  .kWMMMMMMMMWk.         ..      .,cc.    :XMMMMMMMMMWk'       'OWMMMMWk.    :0MMMMKc.cXMMM
MMMMMM0,                                .'.                  :XMMMMMMMMMMNc      'c;ll.       ..     .OMMMMMMMMWx.       :KMMMMMNx.   .oNMMMWO, ;KMMMM
MMMMMMWO'                                                   .xWMMMMMMMMMMMk:.    ,0KOOc              .OMMMMMMMNo.      .oXMMMMMXo.   ;OWMMMXl. 'OMMMMM
MMMMMMMWk.                                                  ,0MMMMMMMMMMMMX0:     .:d00l.            lNMMMMMW0:       ;OWMMMMW0:   'xNMMMNk,  'OWMMMMM
MMMMMMMMWx.                                                 :XMMMMMNNMMMMMMWk.       .',.          'xNMMMMMNx.      ,xNMMMMMNx.  .oXMMMWO:.  'OWMMMMMM
MMMMMMMMMWk.                                                cNMMMMNdxNMMMMMMWO:.                .;xXMMMMMWO:      'dXMMMMMWO;  .oKWMMW0c.   'OWMMMMMMM
MMMMMMMMMMWO,                                               cNWNKk:..:kKNWMMMMWKxc'.         .,oONMMMMMW0c.    .;xXMMMMMWKl..,dXWMMNOc.    ;0WMMMMMMMM
MMMMMMMMMMMMK:                                              cNWN0o'  'oONWMMMMMMMWN0dc;''';lxKNMMMMMMW0l.    .cONMMMMMWKl'.ckNMMMNk;.     cKMMMMMMMMMM
MMMMMMMMMMMMMXo.                                            cNMMMMKccKMMMMMMMMMMMMMMMMWNNNWMMMMMMMMNOc.   .:xXWMMMMMWOl;:dKWMMW0o'      .dNMMMMMMMMMMM
MMMMMMMMMMMMMMWk'                                           lNMMMMMXXMMMMMMMMMMMMMMMMMMMMMMMMMMMWKx;. .,lkXWMMMMMWXOdoxKWMMWKd;.       ;OWMMMMMMMMMMMM
MMMMMMMMMMMMMMMMKl.                                         oWMMMMMMMMMMMMMMMMMMMMMMMMMMMMMMMWXkc,';lxKNMMMMMMMWX0OOXWMMN0d;.        .oXMMMMMMMMMMMMMM
MMMMMMMMMMMMMMMMMWk;                                        lXWMMMMMMMMMMMMMMMMMMMMMMMMMMMMWXOxdxOXWMMMMMMMMMMMWNWMMWKxl,.         .:0WMMMMMMMMMMMMMMM
MMMMMMMMMMMMMMMMMMMXd'                                       .:ok0NWMMMMMMMMMMMMMMMMMMMMMMMWNWWMMMMMMMMMMMMMMMMWX0xl,.            ,kNMMMMMMMMMMMMMMMMM
MMMMMMMMMMMMMMMMMMMMMXo.                                         .':ldOKNWMMMMMMMMMMMMMMMMMMMMMMMMMMMMMMMWXKOdc;..              'xNMMMMMMMMMMMMMMMMMMM
MMMMMMMMMMMMMMMMMMMMMMWKo'                                             .';ccldxO0KXXNWWWWWWWWWNNXKKOkxol:,..                  ,xXMMMMMMMMMMMMMMMMMMMMM
MMMMMMMMMMMMMMMMMMMMMMMMWXd,                                                    ....',,,,,,,,,'....                        .;kNMMMMMMMMMMMMMMMMMMMMMMM
MMMMMMMMMMMMMMMMMMMMMMMMMMMNk:.                                                                                          .l0WMMMMMMMMMMMMMMMMMMMMMMMMM
MMMMMMMMMMMMMMMMMMMMMMMMMMMMMWKo,.                                                                                    .:xXWMMMMMMMMMMMMMMMMMMMMMMMMMMM
MMMMMMMMMMMMMMMMMMMMMMMMMMMMMMMMNOo,                                                                               .;dKWMMMMMMMMMMMMMMMMMMMMMMMMMMMMMM
MMMMMMMMMMMMMMMMMMMMMMMMMMMMMMMMMMMNOo;.                                                                        .:xKWMMMMMMMMMMMMMMMMMMMMMMMMMMMMMMMMM
MMMMMMMMMMMMMMMMMMMMMMMMMMMMMMMMMMMMMMWKxc'.                                                                .;oOXWMMMMMMMMMMMMMMMMMMMMMMMMMMMMMMMMMMMM
MMMMMMMMMMMMMMMMMMMMMMMMMMMMMMMMMMMMMMMMMMN0xl;.                                                       .':oOXWMMMMMMMMMMMMMMMMMMMMMMMMMMMMMMMMMMMMMMMM
MMMMMMMMMMMMMMMMMMMMMMMMMMMMMMMMMMMMMMMMMMMMMMWXOdl;'.                                           ..,cokKNWMMMMMMMMMMMMMMMMMMMMMMMMMMMMMMMMMMMMMMMMMMMM
MMMMMMMMMMMMMMMMMMMMMMMMMMMMMMMMMMMMMMMMMMMMMMMMMMMWNKOxoc;,...                          ..';:ldk0XWMMMMMMMMMMMMMMMMMMMMMMMMMMMMMMMMMMMMMMMMMMMMMMMMMM
MMMMMMMMMMMMMMMMMMMMMMMMMMMMMMMMMMMMMMMMMMMMMMMMMMMMMMMMMMWWXK0Okxdollcccc:::cccclloddkO0KXNWMMMMMMMMMMMMMMMMMMMMMMMMMMMMMMMMMMMMMMMMMMMMMMMMMMMMMMMMM
MMMMMMMMMMMMMMMMMMMMMMMMMMMMMMMMMMMMMMMMMMMMMMMMMMMMMMMMMMMMMMMMMMMMMMMMMMMMMMMMMMMMMMMMMMMMMMMMMMMMMMMMMMMMMMMMMMMMMMMMMMMMMMMMMMMMMMMMMMMMMMMMMMMMMM
    ███╗   ███╗ ██████╗  ██████╗ ███╗   ██╗██╗███████╗██╗    ██╗ █████╗ ██████╗ ███████╗██╗  ██╗ ██████╗██╗  ██╗ █████╗ ███╗   ██╗ ██████╗ ███████╗
    ████╗ ████║██╔═══██╗██╔═══██╗████╗  ██║██║██╔════╝██║    ██║██╔══██╗██╔══██╗██╔════╝╚██╗██╔╝██╔════╝██║  ██║██╔══██╗████╗  ██║██╔════╝ ██╔════╝
    ██╔████╔██║██║   ██║██║   ██║██╔██╗ ██║██║███████╗██║ █╗ ██║███████║██████╔╝█████╗   ╚███╔╝ ██║     ███████║███████║██╔██╗ ██║██║  ███╗█████╗
    ██║╚██╔╝██║██║   ██║██║   ██║██║╚██╗██║██║╚════██║██║███╗██║██╔══██║██╔═══╝ ██╔══╝   ██╔██╗ ██║     ██╔══██║██╔══██║██║╚██╗██║██║   ██║██╔══╝
    ██║ ╚═╝ ██║╚██████╔╝╚██████╔╝██║ ╚████║██║███████║╚███╔███╔╝██║  ██║██║██╗  ███████╗██╔╝ ██╗╚██████╗██║  ██║██║  ██║██║ ╚████║╚██████╔╝███████╗
    ╚═╝     ╚═╝ ╚═════╝  ╚═════╝ ╚═╝  ╚═══╝╚═╝╚══════╝ ╚══╝╚══╝ ╚═╝  ╚═╝╚═╝╚═╝  ╚══════╝╚═╝  ╚═╝ ╚═════╝╚═╝  ╚═╝╚═╝  ╚═╝╚═╝  ╚═══╝ ╚═════╝ ╚══════╝

                                                    ██████╗ ██╗   ██╗     ██╗██╗███╗   ██╗ ██████╗██╗  ██╗
                                                    ██╔══██╗╚██╗ ██╔╝    ███║██║████╗  ██║██╔════╝██║  ██║
                                                    ██████╔╝ ╚████╔╝     ╚██║██║██╔██╗ ██║██║     ███████║
                                                    ██╔══██╗  ╚██╔╝       ██║██║██║╚██╗██║██║     ██╔══██║
                                                    ██████╔╝   ██║        ██║██║██║ ╚████║╚██████╗██║  ██║
                                                    ╚═════╝    ╚═╝        ╚═╝╚═╝╚═╝  ╚═══╝ ╚═════╝╚═╝  ╚═╝
*/
// File: @openzeppelin/contracts/GSN/Context.sol

// SPDX-License-Identifier: MIT

pragma solidity ^0.6.0;

/*
 * @dev Provides information about the current execution context, including the
 * sender of the transaction and its data. While these are generally available
 * via msg.sender and msg.data, they should not be accessed in such a direct
 * manner, since when dealing with GSN meta-transactions the account sending and
 * paying for execution may not be the actual sender (as far as an application
 * is concerned).
 *
 * This contract is only required for intermediate, library-like contracts.
 */
abstract contract Context {
    function _msgSender() internal view virtual returns (address payable) {
        return msg.sender;
    }

    function _msgData() internal view virtual returns (bytes memory) {
        this; // silence state mutability warning without generating bytecode - see https://github.com/ethereum/solidity/issues/2691
        return msg.data;
    }
}

// File: @openzeppelin/contracts/access/Ownable.sol


pragma solidity ^0.6.0;

/**
 * @dev Contract module which provides a basic access control mechanism, where
 * there is an account (an owner) that can be granted exclusive access to
 * specific functions.
 *
 * By default, the owner account will be the one that deploys the contract. This
 * can later be changed with {transferOwnership}.
 *
 * This module is used through inheritance. It will make available the modifier
 * `onlyOwner`, which can be applied to your functions to restrict their use to
 * the owner.
 */
contract Ownable is Context {
    address private _owner;

    event OwnershipTransferred(address indexed previousOwner, address indexed newOwner);

    /**
     * @dev Initializes the contract setting the deployer as the initial owner.
     */
    constructor () internal {
        address msgSender = _msgSender();
        _owner = msgSender;
        emit OwnershipTransferred(address(0), msgSender);
    }

    /**
     * @dev Returns the address of the current owner.
     */
    function owner() public view returns (address) {
        return _owner;
    }

    /**
     * @dev Throws if called by any account other than the owner.
     */
    modifier onlyOwner() {
        require(_owner == _msgSender(), "Ownable: caller is not the owner");
        _;
    }

    /**
     * @dev Leaves the contract without owner. It will not be possible to call
     * `onlyOwner` functions anymore. Can only be called by the current owner.
     *
     * NOTE: Renouncing ownership will leave the contract without an owner,
     * thereby removing any functionality that is only available to the owner.
     */
    function renounceOwnership() public virtual onlyOwner {
        emit OwnershipTransferred(_owner, address(0));
        _owner = address(0);
    }

    /**
     * @dev Transfers ownership of the contract to a new account (`newOwner`).
     * Can only be called by the current owner.
     */
    function transferOwnership(address newOwner) public virtual onlyOwner {
        require(newOwner != address(0), "Ownable: new owner is the zero address");
        emit OwnershipTransferred(_owner, newOwner);
        _owner = newOwner;
    }
}

// File: @openzeppelin/contracts/utils/ReentrancyGuard.sol


pragma solidity ^0.6.0;

/**
 * @dev Contract module that helps prevent reentrant calls to a function.
 *
 * Inheriting from `ReentrancyGuard` will make the {nonReentrant} modifier
 * available, which can be applied to functions to make sure there are no nested
 * (reentrant) calls to them.
 *
 * Note that because there is a single `nonReentrant` guard, functions marked as
 * `nonReentrant` may not call one another. This can be worked around by making
 * those functions `private`, and then adding `external` `nonReentrant` entry
 * points to them.
 *
 * TIP: If you would like to learn more about reentrancy and alternative ways
 * to protect against it, check out our blog post
 * https://blog.openzeppelin.com/reentrancy-after-istanbul/[Reentrancy After Istanbul].
 */
contract ReentrancyGuard {
    // Booleans are more expensive than uint256 or any type that takes up a full
    // word because each write operation emits an extra SLOAD to first read the
    // slot's contents, replace the bits taken up by the boolean, and then write
    // back. This is the compiler's defense against contract upgrades and
    // pointer aliasing, and it cannot be disabled.

    // The values being non-zero value makes deployment a bit more expensive,
    // but in exchange the refund on every call to nonReentrant will be lower in
    // amount. Since refunds are capped to a percentage of the total
    // transaction's gas, it is best to keep them low in cases like this one, to
    // increase the likelihood of the full refund coming into effect.
    uint256 private constant _NOT_ENTERED = 1;
    uint256 private constant _ENTERED = 2;

    uint256 private _status;

    constructor () internal {
        _status = _NOT_ENTERED;
    }

    /**
     * @dev Prevents a contract from calling itself, directly or indirectly.
     * Calling a `nonReentrant` function from another `nonReentrant`
     * function is not supported. It is possible to prevent this from happening
     * by making the `nonReentrant` function external, and make it call a
     * `private` function that does the actual work.
     */
    modifier nonReentrant() {
        // On the first call to nonReentrant, _notEntered will be true
        require(_status != _ENTERED, "ReentrancyGuard: reentrant call");

        // Any calls to nonReentrant after this point will fail
        _status = _ENTERED;

        _;

        // By storing the original value once again, a refund is triggered (see
        // https://eips.ethereum.org/EIPS/eip-2200)
        _status = _NOT_ENTERED;
    }
}

// File: @openzeppelin/contracts/math/Math.sol


pragma solidity ^0.6.0;

/**
 * @dev Standard math utilities missing in the Solidity language.
 */
library Math {
    /**
     * @dev Returns the largest of two numbers.
     */
    function max(uint256 a, uint256 b) internal pure returns (uint256) {
        return a >= b ? a : b;
    }

    /**
     * @dev Returns the smallest of two numbers.
     */
    function min(uint256 a, uint256 b) internal pure returns (uint256) {
        return a < b ? a : b;
    }

    /**
     * @dev Returns the average of two numbers. The result is rounded towards
     * zero.
     */
    function average(uint256 a, uint256 b) internal pure returns (uint256) {
        // (a + b) / 2 can overflow, so we distribute
        return (a / 2) + (b / 2) + ((a % 2 + b % 2) / 2);
    }
}

// File: @openzeppelin/contracts/math/SafeMath.sol


pragma solidity ^0.6.0;

/**
 * @dev Wrappers over Solidity's arithmetic operations with added overflow
 * checks.
 *
 * Arithmetic operations in Solidity wrap on overflow. This can easily result
 * in bugs, because programmers usually assume that an overflow raises an
 * error, which is the standard behavior in high level programming languages.
 * `SafeMath` restores this intuition by reverting the transaction when an
 * operation overflows.
 *
 * Using this library instead of the unchecked operations eliminates an entire
 * class of bugs, so it's recommended to use it always.
 */
library SafeMath {
    /**
     * @dev Returns the addition of two unsigned integers, reverting on
     * overflow.
     *
     * Counterpart to Solidity's `+` operator.
     *
     * Requirements:
     *
     * - Addition cannot overflow.
     */
    function add(uint256 a, uint256 b) internal pure returns (uint256) {
        uint256 c = a + b;
        require(c >= a, "SafeMath: addition overflow");

        return c;
    }

    /**
     * @dev Returns the subtraction of two unsigned integers, reverting on
     * overflow (when the result is negative).
     *
     * Counterpart to Solidity's `-` operator.
     *
     * Requirements:
     *
     * - Subtraction cannot overflow.
     */
    function sub(uint256 a, uint256 b) internal pure returns (uint256) {
        return sub(a, b, "SafeMath: subtraction overflow");
    }

    /**
     * @dev Returns the subtraction of two unsigned integers, reverting with custom message on
     * overflow (when the result is negative).
     *
     * Counterpart to Solidity's `-` operator.
     *
     * Requirements:
     *
     * - Subtraction cannot overflow.
     */
    function sub(uint256 a, uint256 b, string memory errorMessage) internal pure returns (uint256) {
        require(b <= a, errorMessage);
        uint256 c = a - b;

        return c;
    }

    /**
     * @dev Returns the multiplication of two unsigned integers, reverting on
     * overflow.
     *
     * Counterpart to Solidity's `*` operator.
     *
     * Requirements:
     *
     * - Multiplication cannot overflow.
     */
    function mul(uint256 a, uint256 b) internal pure returns (uint256) {
        // Gas optimization: this is cheaper than requiring 'a' not being zero, but the
        // benefit is lost if 'b' is also tested.
        // See: https://github.com/OpenZeppelin/openzeppelin-contracts/pull/522
        if (a == 0) {
            return 0;
        }

        uint256 c = a * b;
        require(c / a == b, "SafeMath: multiplication overflow");

        return c;
    }

    /**
     * @dev Returns the integer division of two unsigned integers. Reverts on
     * division by zero. The result is rounded towards zero.
     *
     * Counterpart to Solidity's `/` operator. Note: this function uses a
     * `revert` opcode (which leaves remaining gas untouched) while Solidity
     * uses an invalid opcode to revert (consuming all remaining gas).
     *
     * Requirements:
     *
     * - The divisor cannot be zero.
     */
    function div(uint256 a, uint256 b) internal pure returns (uint256) {
        return div(a, b, "SafeMath: division by zero");
    }

    /**
     * @dev Returns the integer division of two unsigned integers. Reverts with custom message on
     * division by zero. The result is rounded towards zero.
     *
     * Counterpart to Solidity's `/` operator. Note: this function uses a
     * `revert` opcode (which leaves remaining gas untouched) while Solidity
     * uses an invalid opcode to revert (consuming all remaining gas).
     *
     * Requirements:
     *
     * - The divisor cannot be zero.
     */
    function div(uint256 a, uint256 b, string memory errorMessage) internal pure returns (uint256) {
        require(b > 0, errorMessage);
        uint256 c = a / b;
        // assert(a == b * c + a % b); // There is no case in which this doesn't hold

        return c;
    }

    /**
     * @dev Returns the remainder of dividing two unsigned integers. (unsigned integer modulo),
     * Reverts when dividing by zero.
     *
     * Counterpart to Solidity's `%` operator. This function uses a `revert`
     * opcode (which leaves remaining gas untouched) while Solidity uses an
     * invalid opcode to revert (consuming all remaining gas).
     *
     * Requirements:
     *
     * - The divisor cannot be zero.
     */
    function mod(uint256 a, uint256 b) internal pure returns (uint256) {
        return mod(a, b, "SafeMath: modulo by zero");
    }

    /**
     * @dev Returns the remainder of dividing two unsigned integers. (unsigned integer modulo),
     * Reverts with custom message when dividing by zero.
     *
     * Counterpart to Solidity's `%` operator. This function uses a `revert`
     * opcode (which leaves remaining gas untouched) while Solidity uses an
     * invalid opcode to revert (consuming all remaining gas).
     *
     * Requirements:
     *
     * - The divisor cannot be zero.
     */
    function mod(uint256 a, uint256 b, string memory errorMessage) internal pure returns (uint256) {
        require(b != 0, errorMessage);
        return a % b;
    }
}

// File: @openzeppelin/contracts/token/ERC20/IERC20.sol


pragma solidity ^0.6.0;

/**
 * @dev Interface of the ERC20 standard as defined in the EIP.
 */
interface IERC20 {
    /**
     * @dev Returns the amount of tokens in existence.
     */
    function totalSupply() external view returns (uint256);

    /**
     * @dev Returns the amount of tokens owned by `account`.
     */
    function balanceOf(address account) external view returns (uint256);

    /**
     * @dev Moves `amount` tokens from the caller's account to `recipient`.
     *
     * Returns a boolean value indicating whether the operation succeeded.
     *
     * Emits a {Transfer} event.
     */
    function transfer(address recipient, uint256 amount) external returns (bool);

    /**
     * @dev Returns the remaining number of tokens that `spender` will be
     * allowed to spend on behalf of `owner` through {transferFrom}. This is
     * zero by default.
     *
     * This value changes when {approve} or {transferFrom} are called.
     */
    function allowance(address owner, address spender) external view returns (uint256);

    /**
     * @dev Sets `amount` as the allowance of `spender` over the caller's tokens.
     *
     * Returns a boolean value indicating whether the operation succeeded.
     *
     * IMPORTANT: Beware that changing an allowance with this method brings the risk
     * that someone may use both the old and the new allowance by unfortunate
     * transaction ordering. One possible solution to mitigate this race
     * condition is to first reduce the spender's allowance to 0 and set the
     * desired value afterwards:
     * https://github.com/ethereum/EIPs/issues/20#issuecomment-263524729
     *
     * Emits an {Approval} event.
     */
    function approve(address spender, uint256 amount) external returns (bool);

    /**
     * @dev Moves `amount` tokens from `sender` to `recipient` using the
     * allowance mechanism. `amount` is then deducted from the caller's
     * allowance.
     *
     * Returns a boolean value indicating whether the operation succeeded.
     *
     * Emits a {Transfer} event.
     */
    function transferFrom(address sender, address recipient, uint256 amount) external returns (bool);

    /**
     * @dev Emitted when `value` tokens are moved from one account (`from`) to
     * another (`to`).
     *
     * Note that `value` may be zero.
     */
    event Transfer(address indexed from, address indexed to, uint256 value);

    /**
     * @dev Emitted when the allowance of a `spender` for an `owner` is set by
     * a call to {approve}. `value` is the new allowance.
     */
    event Approval(address indexed owner, address indexed spender, uint256 value);
}

// File: @openzeppelin/contracts/utils/Address.sol


pragma solidity ^0.6.2;

/**
 * @dev Collection of functions related to the address type
 */
library Address {
    /**
     * @dev Returns true if `account` is a contract.
     *
     * [IMPORTANT]
     * ====
     * It is unsafe to assume that an address for which this function returns
     * false is an externally-owned account (EOA) and not a contract.
     *
     * Among others, `isContract` will return false for the following
     * types of addresses:
     *
     *  - an externally-owned account
     *  - a contract in construction
     *  - an address where a contract will be created
     *  - an address where a contract lived, but was destroyed
     * ====
     */
    function isContract(address account) internal view returns (bool) {
        // According to EIP-1052, 0x0 is the value returned for not-yet created accounts
        // and 0xc5d2460186f7233c927e7db2dcc703c0e500b653ca82273b7bfad8045d85a470 is returned
        // for accounts without code, i.e. `keccak256('')`
        bytes32 codehash;
        bytes32 accountHash = 0xc5d2460186f7233c927e7db2dcc703c0e500b653ca82273b7bfad8045d85a470;
        // solhint-disable-next-line no-inline-assembly
        assembly { codehash := extcodehash(account) }
        return (codehash != accountHash && codehash != 0x0);
    }

    /**
     * @dev Replacement for Solidity's `transfer`: sends `amount` wei to
     * `recipient`, forwarding all available gas and reverting on errors.
     *
     * https://eips.ethereum.org/EIPS/eip-1884[EIP1884] increases the gas cost
     * of certain opcodes, possibly making contracts go over the 2300 gas limit
     * imposed by `transfer`, making them unable to receive funds via
     * `transfer`. {sendValue} removes this limitation.
     *
     * https://diligence.consensys.net/posts/2019/09/stop-using-soliditys-transfer-now/[Learn more].
     *
     * IMPORTANT: because control is transferred to `recipient`, care must be
     * taken to not create reentrancy vulnerabilities. Consider using
     * {ReentrancyGuard} or the
     * https://solidity.readthedocs.io/en/v0.5.11/security-considerations.html#use-the-checks-effects-interactions-pattern[checks-effects-interactions pattern].
     */
    function sendValue(address payable recipient, uint256 amount) internal {
        require(address(this).balance >= amount, "Address: insufficient balance");

        // solhint-disable-next-line avoid-low-level-calls, avoid-call-value
        (bool success, ) = recipient.call{ value: amount }("");
        require(success, "Address: unable to send value, recipient may have reverted");
    }

    /**
     * @dev Performs a Solidity function call using a low level `call`. A
     * plain`call` is an unsafe replacement for a function call: use this
     * function instead.
     *
     * If `target` reverts with a revert reason, it is bubbled up by this
     * function (like regular Solidity function calls).
     *
     * Returns the raw returned data. To convert to the expected return value,
     * use https://solidity.readthedocs.io/en/latest/units-and-global-variables.html?highlight=abi.decode#abi-encoding-and-decoding-functions[`abi.decode`].
     *
     * Requirements:
     *
     * - `target` must be a contract.
     * - calling `target` with `data` must not revert.
     *
     * _Available since v3.1._
     */
    function functionCall(address target, bytes memory data) internal returns (bytes memory) {
      return functionCall(target, data, "Address: low-level call failed");
    }

    /**
     * @dev Same as {xref-Address-functionCall-address-bytes-}[`functionCall`], but with
     * `errorMessage` as a fallback revert reason when `target` reverts.
     *
     * _Available since v3.1._
     */
    function functionCall(address target, bytes memory data, string memory errorMessage) internal returns (bytes memory) {
        return _functionCallWithValue(target, data, 0, errorMessage);
    }

    /**
     * @dev Same as {xref-Address-functionCall-address-bytes-}[`functionCall`],
     * but also transferring `value` wei to `target`.
     *
     * Requirements:
     *
     * - the calling contract must have an ETH balance of at least `value`.
     * - the called Solidity function must be `payable`.
     *
     * _Available since v3.1._
     */
    function functionCallWithValue(address target, bytes memory data, uint256 value) internal returns (bytes memory) {
        return functionCallWithValue(target, data, value, "Address: low-level call with value failed");
    }

    /**
     * @dev Same as {xref-Address-functionCallWithValue-address-bytes-uint256-}[`functionCallWithValue`], but
     * with `errorMessage` as a fallback revert reason when `target` reverts.
     *
     * _Available since v3.1._
     */
    function functionCallWithValue(address target, bytes memory data, uint256 value, string memory errorMessage) internal returns (bytes memory) {
        require(address(this).balance >= value, "Address: insufficient balance for call");
        return _functionCallWithValue(target, data, value, errorMessage);
    }

    function _functionCallWithValue(address target, bytes memory data, uint256 weiValue, string memory errorMessage) private returns (bytes memory) {
        require(isContract(target), "Address: call to non-contract");

        // solhint-disable-next-line avoid-low-level-calls
        (bool success, bytes memory returndata) = target.call{ value: weiValue }(data);
        if (success) {
            return returndata;
        } else {
            // Look for revert reason and bubble it up if present
            if (returndata.length > 0) {
                // The easiest way to bubble the revert reason is using memory via assembly

                // solhint-disable-next-line no-inline-assembly
                assembly {
                    let returndata_size := mload(returndata)
                    revert(add(32, returndata), returndata_size)
                }
            } else {
                revert(errorMessage);
            }
        }
    }
}

// File: @openzeppelin/contracts/token/ERC20/ERC20.sol


pragma solidity ^0.6.0;





/**
 * @dev Implementation of the {IERC20} interface.
 *
 * This implementation is agnostic to the way tokens are created. This means
 * that a supply mechanism has to be added in a derived contract using {_mint}.
 * For a generic mechanism see {ERC20PresetMinterPauser}.
 *
 * TIP: For a detailed writeup see our guide
 * https://forum.zeppelin.solutions/t/how-to-implement-erc20-supply-mechanisms/226[How
 * to implement supply mechanisms].
 *
 * We have followed general OpenZeppelin guidelines: functions revert instead
 * of returning `false` on failure. This behavior is nonetheless conventional
 * and does not conflict with the expectations of ERC20 applications.
 *
 * Additionally, an {Approval} event is emitted on calls to {transferFrom}.
 * This allows applications to reconstruct the allowance for all accounts just
 * by listening to said events. Other implementations of the EIP may not emit
 * these events, as it isn't required by the specification.
 *
 * Finally, the non-standard {decreaseAllowance} and {increaseAllowance}
 * functions have been added to mitigate the well-known issues around setting
 * allowances. See {IERC20-approve}.
 */
contract ERC20 is Context, IERC20 {
    using SafeMath for uint256;
    using Address for address;

    mapping (address => uint256) private _balances;

    mapping (address => mapping (address => uint256)) private _allowances;

    uint256 private _totalSupply;

    string private _name;
    string private _symbol;
    uint8 private _decimals;

    /**
     * @dev Sets the values for {name} and {symbol}, initializes {decimals} with
     * a default value of 18.
     *
     * To select a different value for {decimals}, use {_setupDecimals}.
     *
     * All three of these values are immutable: they can only be set once during
     * construction.
     */
    constructor (string memory name, string memory symbol) public {
        _name = name;
        _symbol = symbol;
        _decimals = 18;
    }

    /**
     * @dev Returns the name of the token.
     */
    function name() public view returns (string memory) {
        return _name;
    }

    /**
     * @dev Returns the symbol of the token, usually a shorter version of the
     * name.
     */
    function symbol() public view returns (string memory) {
        return _symbol;
    }

    /**
     * @dev Returns the number of decimals used to get its user representation.
     * For example, if `decimals` equals `2`, a balance of `505` tokens should
     * be displayed to a user as `5,05` (`505 / 10 ** 2`).
     *
     * Tokens usually opt for a value of 18, imitating the relationship between
     * Ether and Wei. This is the value {ERC20} uses, unless {_setupDecimals} is
     * called.
     *
     * NOTE: This information is only used for _display_ purposes: it in
     * no way affects any of the arithmetic of the contract, including
     * {IERC20-balanceOf} and {IERC20-transfer}.
     */
    function decimals() public view returns (uint8) {
        return _decimals;
    }

    /**
     * @dev See {IERC20-totalSupply}.
     */
    function totalSupply() public view override returns (uint256) {
        return _totalSupply;
    }

    /**
     * @dev See {IERC20-balanceOf}.
     */
    function balanceOf(address account) public view override returns (uint256) {
        return _balances[account];
    }

    /**
     * @dev See {IERC20-transfer}.
     *
     * Requirements:
     *
     * - `recipient` cannot be the zero address.
     * - the caller must have a balance of at least `amount`.
     */
    function transfer(address recipient, uint256 amount) public virtual override returns (bool) {
        _transfer(_msgSender(), recipient, amount);
        return true;
    }

    /**
     * @dev See {IERC20-allowance}.
     */
    function allowance(address owner, address spender) public view virtual override returns (uint256) {
        return _allowances[owner][spender];
    }

    /**
     * @dev See {IERC20-approve}.
     *
     * Requirements:
     *
     * - `spender` cannot be the zero address.
     */
    function approve(address spender, uint256 amount) public virtual override returns (bool) {
        _approve(_msgSender(), spender, amount);
        return true;
    }

    /**
     * @dev See {IERC20-transferFrom}.
     *
     * Emits an {Approval} event indicating the updated allowance. This is not
     * required by the EIP. See the note at the beginning of {ERC20};
     *
     * Requirements:
     * - `sender` and `recipient` cannot be the zero address.
     * - `sender` must have a balance of at least `amount`.
     * - the caller must have allowance for ``sender``'s tokens of at least
     * `amount`.
     */
    function transferFrom(address sender, address recipient, uint256 amount) public virtual override returns (bool) {
        _transfer(sender, recipient, amount);
        _approve(sender, _msgSender(), _allowances[sender][_msgSender()].sub(amount, "ERC20: transfer amount exceeds allowance"));
        return true;
    }

    /**
     * @dev Atomically increases the allowance granted to `spender` by the caller.
     *
     * This is an alternative to {approve} that can be used as a mitigation for
     * problems described in {IERC20-approve}.
     *
     * Emits an {Approval} event indicating the updated allowance.
     *
     * Requirements:
     *
     * - `spender` cannot be the zero address.
     */
    function increaseAllowance(address spender, uint256 addedValue) public virtual returns (bool) {
        _approve(_msgSender(), spender, _allowances[_msgSender()][spender].add(addedValue));
        return true;
    }

    /**
     * @dev Atomically decreases the allowance granted to `spender` by the caller.
     *
     * This is an alternative to {approve} that can be used as a mitigation for
     * problems described in {IERC20-approve}.
     *
     * Emits an {Approval} event indicating the updated allowance.
     *
     * Requirements:
     *
     * - `spender` cannot be the zero address.
     * - `spender` must have allowance for the caller of at least
     * `subtractedValue`.
     */
    function decreaseAllowance(address spender, uint256 subtractedValue) public virtual returns (bool) {
        _approve(_msgSender(), spender, _allowances[_msgSender()][spender].sub(subtractedValue, "ERC20: decreased allowance below zero"));
        return true;
    }

    /**
     * @dev Moves tokens `amount` from `sender` to `recipient`.
     *
     * This is internal function is equivalent to {transfer}, and can be used to
     * e.g. implement automatic token fees, slashing mechanisms, etc.
     *
     * Emits a {Transfer} event.
     *
     * Requirements:
     *
     * - `sender` cannot be the zero address.
     * - `recipient` cannot be the zero address.
     * - `sender` must have a balance of at least `amount`.
     */
    function _transfer(address sender, address recipient, uint256 amount) internal virtual {
        require(sender != address(0), "ERC20: transfer from the zero address");
        require(recipient != address(0), "ERC20: transfer to the zero address");

        _beforeTokenTransfer(sender, recipient, amount);

        _balances[sender] = _balances[sender].sub(amount, "ERC20: transfer amount exceeds balance");
        _balances[recipient] = _balances[recipient].add(amount);
        emit Transfer(sender, recipient, amount);
    }

    /** @dev Creates `amount` tokens and assigns them to `account`, increasing
     * the total supply.
     *
     * Emits a {Transfer} event with `from` set to the zero address.
     *
     * Requirements
     *
     * - `to` cannot be the zero address.
     */
    function _mint(address account, uint256 amount) internal virtual {
        require(account != address(0), "ERC20: mint to the zero address");

        _beforeTokenTransfer(address(0), account, amount);

        _totalSupply = _totalSupply.add(amount);
        _balances[account] = _balances[account].add(amount);
        emit Transfer(address(0), account, amount);
    }

    /**
     * @dev Destroys `amount` tokens from `account`, reducing the
     * total supply.
     *
     * Emits a {Transfer} event with `to` set to the zero address.
     *
     * Requirements
     *
     * - `account` cannot be the zero address.
     * - `account` must have at least `amount` tokens.
     */
    function _burn(address account, uint256 amount) internal virtual {
        require(account != address(0), "ERC20: burn from the zero address");

        _beforeTokenTransfer(account, address(0), amount);

        _balances[account] = _balances[account].sub(amount, "ERC20: burn amount exceeds balance");
        _totalSupply = _totalSupply.sub(amount);
        emit Transfer(account, address(0), amount);
    }

    /**
     * @dev Sets `amount` as the allowance of `spender` over the `owner`s tokens.
     *
     * This is internal function is equivalent to `approve`, and can be used to
     * e.g. set automatic allowances for certain subsystems, etc.
     *
     * Emits an {Approval} event.
     *
     * Requirements:
     *
     * - `owner` cannot be the zero address.
     * - `spender` cannot be the zero address.
     */
    function _approve(address owner, address spender, uint256 amount) internal virtual {
        require(owner != address(0), "ERC20: approve from the zero address");
        require(spender != address(0), "ERC20: approve to the zero address");

        _allowances[owner][spender] = amount;
        emit Approval(owner, spender, amount);
    }

    /**
     * @dev Sets {decimals} to a value other than the default one of 18.
     *
     * WARNING: This function should only be called from the constructor. Most
     * applications that interact with token contracts will not expect
     * {decimals} to ever change, and may work incorrectly if it does.
     */
    function _setupDecimals(uint8 decimals_) internal {
        _decimals = decimals_;
    }

    /**
     * @dev Hook that is called before any transfer of tokens. This includes
     * minting and burning.
     *
     * Calling conditions:
     *
     * - when `from` and `to` are both non-zero, `amount` of ``from``'s tokens
     * will be to transferred to `to`.
     * - when `from` is zero, `amount` tokens will be minted for `to`.
     * - when `to` is zero, `amount` of ``from``'s tokens will be burned.
     * - `from` and `to` are never both zero.
     *
     * To learn more about hooks, head to xref:ROOT:extending-contracts.adoc#using-hooks[Using Hooks].
     */
    function _beforeTokenTransfer(address from, address to, uint256 amount) internal virtual { }
}

// File: @openzeppelin/contracts/token/ERC20/SafeERC20.sol


pragma solidity ^0.6.0;




/**
 * @title SafeERC20
 * @dev Wrappers around ERC20 operations that throw on failure (when the token
 * contract returns false). Tokens that return no value (and instead revert or
 * throw on failure) are also supported, non-reverting calls are assumed to be
 * successful.
 * To use this library you can add a `using SafeERC20 for IERC20;` statement to your contract,
 * which allows you to call the safe operations as `token.safeTransfer(...)`, etc.
 */
library SafeERC20 {
    using SafeMath for uint256;
    using Address for address;

    function safeTransfer(IERC20 token, address to, uint256 value) internal {
        _callOptionalReturn(token, abi.encodeWithSelector(token.transfer.selector, to, value));
    }

    function safeTransferFrom(IERC20 token, address from, address to, uint256 value) internal {
        _callOptionalReturn(token, abi.encodeWithSelector(token.transferFrom.selector, from, to, value));
    }

    /**
     * @dev Deprecated. This function has issues similar to the ones found in
     * {IERC20-approve}, and its usage is discouraged.
     *
     * Whenever possible, use {safeIncreaseAllowance} and
     * {safeDecreaseAllowance} instead.
     */
    function safeApprove(IERC20 token, address spender, uint256 value) internal {
        // safeApprove should only be called when setting an initial allowance,
        // or when resetting it to zero. To increase and decrease it, use
        // 'safeIncreaseAllowance' and 'safeDecreaseAllowance'
        // solhint-disable-next-line max-line-length
        require((value == 0) || (token.allowance(address(this), spender) == 0),
            "SafeERC20: approve from non-zero to non-zero allowance"
        );
        _callOptionalReturn(token, abi.encodeWithSelector(token.approve.selector, spender, value));
    }

    function safeIncreaseAllowance(IERC20 token, address spender, uint256 value) internal {
        uint256 newAllowance = token.allowance(address(this), spender).add(value);
        _callOptionalReturn(token, abi.encodeWithSelector(token.approve.selector, spender, newAllowance));
    }

    function safeDecreaseAllowance(IERC20 token, address spender, uint256 value) internal {
        uint256 newAllowance = token.allowance(address(this), spender).sub(value, "SafeERC20: decreased allowance below zero");
        _callOptionalReturn(token, abi.encodeWithSelector(token.approve.selector, spender, newAllowance));
    }

    /**
     * @dev Imitates a Solidity high-level call (i.e. a regular function call to a contract), relaxing the requirement
     * on the return value: the return value is optional (but if data is returned, it must not be false).
     * @param token The token targeted by the call.
     * @param data The call data (encoded using abi.encode or one of its variants).
     */
    function _callOptionalReturn(IERC20 token, bytes memory data) private {
        // We need to perform a low level call here, to bypass Solidity's return data size checking mechanism, since
        // we're implementing it ourselves. We use {Address.functionCall} to perform this call, which verifies that
        // the target address contains contract code and also asserts for success in the low-level call.

        bytes memory returndata = address(token).functionCall(data, "SafeERC20: low-level call failed");
        if (returndata.length > 0) { // Return data is optional
            // solhint-disable-next-line max-line-length
            require(abi.decode(returndata, (bool)), "SafeERC20: ERC20 operation did not succeed");
        }
    }
}

// File: contracts/libraries/UniERC20.sol


pragma solidity ^0.6.0;





library UniERC20 {
    using SafeMath for uint256;
    using SafeERC20 for IERC20;

    function isETH(IERC20 token) internal pure returns(bool) {
        return (address(token) == address(0));
    }

    function uniBalanceOf(IERC20 token, address account) internal view returns (uint256) {
        if (isETH(token)) {
            return account.balance;
        } else {
            return token.balanceOf(account);
        }
    }

    function uniTransfer(IERC20 token, address payable to, uint256 amount) internal {
        if (amount > 0) {
            if (isETH(token)) {
                to.transfer(amount);
            } else {
                token.safeTransfer(to, amount);
            }
        }
    }

    function uniTransferFromSenderToThis(IERC20 token, uint256 amount) internal {
        if (amount > 0) {
            if (isETH(token)) {
                require(msg.value >= amount, "UniERC20: not enough value");
                if (msg.value > amount) {
                    // Return remainder if exist
                    msg.sender.transfer(msg.value.sub(amount));
                }
            } else {
                token.safeTransferFrom(msg.sender, address(this), amount);
            }
        }
    }

    function uniSymbol(IERC20 token) internal view returns(string memory) {
        if (isETH(token)) {
            return "ETH";
        }

        (bool success, bytes memory data) = address(token).staticcall{ gas: 20000 }(
            abi.encodeWithSignature("symbol()")
        );
        if (!success) {
            (success, data) = address(token).staticcall{ gas: 20000 }(
                abi.encodeWithSignature("SYMBOL()")
            );
        }

        if (success && data.length >= 96) {
            (uint256 offset, uint256 len) = abi.decode(data, (uint256, uint256));
            if (offset == 0x20 && len > 0 && len <= 256) {
                return string(abi.decode(data, (bytes)));
            }
        }

        if (success && data.length == 32) {
            uint len = 0;
            while (len < data.length && data[len] >= 0x20 && data[len] <= 0x7E) {
                len++;
            }

            if (len > 0) {
                bytes memory result = new bytes(len);
                for (uint i = 0; i < len; i++) {
                    result[i] = data[i];
                }
                return string(result);
            }
        }

        return _toHex(address(token));
    }

    function _toHex(address account) private pure returns(string memory) {
        return _toHex(abi.encodePacked(account));
    }

    function _toHex(bytes memory data) private pure returns(string memory) {
        bytes memory str = new bytes(2 + data.length * 2);
        str[0] = "0";
        str[1] = "x";
        uint j = 2;
        for (uint i = 0; i < data.length; i++) {
            uint a = uint8(data[i]) >> 4;
            uint b = uint8(data[i]) & 0x0f;
            str[j++] = byte(uint8(a + 48 + (a/10)*39));
            str[j++] = byte(uint8(b + 48 + (b/10)*39));
        }

        return string(str);
    }
}

// File: contracts/libraries/Sqrt.sol


pragma solidity ^0.6.0;


library Sqrt {
    // babylonian method (https://en.wikipedia.org/wiki/Methods_of_computing_square_roots#Babylonian_method)
    function sqrt(uint256 y) internal pure returns (uint256) {
        if (y > 3) {
            uint256 z = y;
            uint256 x = y / 2 + 1;
            while (x < z) {
                z = x;
                x = (y / x + x) / 2;
            }
            return z;
        } else if (y != 0) {
            return 1;
        } else {
            return 0;
        }
    }
}

// File: contracts/Mooniswap.sol


pragma solidity ^0.6.0;









interface IFactory {
    function fee() external view returns(uint256);
}


library VirtualBalance {
    using SafeMath for uint256;

    struct Data {
        uint216 balance;
        uint40 time;
    }

    uint256 public constant DECAY_PERIOD = 5 minutes;

    function set(VirtualBalance.Data storage self, uint256 balance) internal {
        self.balance = uint216(balance);
        self.time = uint40(block.timestamp);
    }

    function update(VirtualBalance.Data storage self, uint256 realBalance) internal {
        set(self, current(self, realBalance));
    }

    function scale(VirtualBalance.Data storage self, uint256 realBalance, uint256 num, uint256 denom) internal {
        set(self, current(self, realBalance).mul(num).add(denom.sub(1)).div(denom));
    }

    function current(VirtualBalance.Data memory self, uint256 realBalance) internal view returns(uint256) {
        uint256 timePassed = Math.min(DECAY_PERIOD, block.timestamp.sub(self.time));
        uint256 timeRemain = DECAY_PERIOD.sub(timePassed);
        return uint256(self.balance).mul(timeRemain).add(
            realBalance.mul(timePassed)
        ).div(DECAY_PERIOD);
    }
}


contract Mooniswap is ERC20, ReentrancyGuard, Ownable {
    using Sqrt for uint256;
    using SafeMath for uint256;
    using UniERC20 for IERC20;
    using VirtualBalance for VirtualBalance.Data;

    struct Balances {
        uint256 src;
        uint256 dst;
    }

    struct SwapVolumes {
        uint128 confirmed;
        uint128 result;
    }

    event Deposited(
        address indexed account,
        uint256 amount
    );

    event Withdrawn(
        address indexed account,
        uint256 amount
    );

    event Swapped(
        address indexed account,
        address indexed src,
        address indexed dst,
        uint256 amount,
        uint256 result,
        uint256 srcBalance,
        uint256 dstBalance,
        uint256 totalSupply,
        address referral
    );

    uint256 public constant REFERRAL_SHARE = 20; // 1/share = 5% of LPs revenue
    uint256 public constant BASE_SUPPLY = 1000;  // Total supply on first deposit
    uint256 public constant FEE_DENOMINATOR = 1e18;

    IFactory public factory;
    IERC20[] public tokens;
    mapping(IERC20 => bool) public isToken;
    mapping(IERC20 => SwapVolumes) public volumes;
    mapping(IERC20 => VirtualBalance.Data) public virtualBalancesForAddition;
    mapping(IERC20 => VirtualBalance.Data) public virtualBalancesForRemoval;

    constructor(IERC20[] memory assets, string memory name, string memory symbol) public ERC20(name, symbol) {
        require(bytes(name).length > 0, "Mooniswap: name is empty");
        require(bytes(symbol).length > 0, "Mooniswap: symbol is empty");
        require(assets.length == 2, "Mooniswap: only 2 tokens allowed");

        factory = IFactory(msg.sender);
        tokens = assets;
        for (uint i = 0; i < assets.length; i++) {
            require(!isToken[assets[i]], "Mooniswap: duplicate tokens");
            isToken[assets[i]] = true;
        }
    }

    function fee() public view returns(uint256) {
        return factory.fee();
    }

    function getTokens() external view returns(IERC20[] memory) {
        return tokens;
    }

    function decayPeriod() external pure returns(uint256) {
        return VirtualBalance.DECAY_PERIOD;
    }

    function getBalanceForAddition(IERC20 token) public view returns(uint256) {
        uint256 balance = token.uniBalanceOf(address(this));
        return Math.max(virtualBalancesForAddition[token].current(balance), balance);
    }

    function getBalanceForRemoval(IERC20 token) public view returns(uint256) {
        uint256 balance = token.uniBalanceOf(address(this));
        return Math.min(virtualBalancesForRemoval[token].current(balance), balance);
    }

    function getReturn(IERC20 src, IERC20 dst, uint256 amount) external view returns(uint256) {
        return _getReturn(src, dst, amount, getBalanceForAddition(src), getBalanceForRemoval(dst));
    }

    function deposit(uint256[] calldata amounts, uint256[] calldata minAmounts) external payable nonReentrant returns(uint256 fairSupply) {
        IERC20[] memory _tokens = tokens;
        require(amounts.length == _tokens.length, "Mooniswap: wrong amounts length");
        require(msg.value == (_tokens[0].isETH() ? amounts[0] : (_tokens[1].isETH() ? amounts[1] : 0)), "Mooniswap: wrong value usage");

        uint256[] memory realBalances = new uint256[](amounts.length);
        for (uint i = 0; i < realBalances.length; i++) {
            realBalances[i] = _tokens[i].uniBalanceOf(address(this)).sub(_tokens[i].isETH() ? msg.value : 0);
        }

        uint256 totalSupply = totalSupply();
        if (totalSupply == 0) {
            fairSupply = BASE_SUPPLY.mul(99);
            _mint(address(this), BASE_SUPPLY); // Donate up to 1%

            // Use the greatest token amount but not less than 99k for the initial supply
            for (uint i = 0; i < amounts.length; i++) {
                fairSupply = Math.max(fairSupply, amounts[i]);
            }
        }
        else {
            // Pre-compute fair supply
            fairSupply = type(uint256).max;
            for (uint i = 0; i < amounts.length; i++) {
                fairSupply = Math.min(fairSupply, totalSupply.mul(amounts[i]).div(realBalances[i]));
            }
        }

        uint256 fairSupplyCached = fairSupply;
        for (uint i = 0; i < amounts.length; i++) {
            require(amounts[i] > 0, "Mooniswap: amount is zero");
            uint256 amount = (totalSupply == 0) ? amounts[i] :
                realBalances[i].mul(fairSupplyCached).add(totalSupply - 1).div(totalSupply);
            require(amount >= minAmounts[i], "Mooniswap: minAmount not reached");

            _tokens[i].uniTransferFromSenderToThis(amount);
            if (totalSupply > 0) {
                uint256 confirmed = _tokens[i].uniBalanceOf(address(this)).sub(realBalances[i]);
                fairSupply = Math.min(fairSupply, totalSupply.mul(confirmed).div(realBalances[i]));
            }
        }

        if (totalSupply > 0) {
            for (uint i = 0; i < amounts.length; i++) {
                virtualBalancesForRemoval[_tokens[i]].scale(realBalances[i], totalSupply.add(fairSupply), totalSupply);
                virtualBalancesForAddition[_tokens[i]].scale(realBalances[i], totalSupply.add(fairSupply), totalSupply);
            }
        }

        require(fairSupply > 0, "Mooniswap: result is not enough");
        _mint(msg.sender, fairSupply);

        emit Deposited(msg.sender, fairSupply);
    }

    function withdraw(uint256 amount, uint256[] memory minReturns) external nonReentrant {
        uint256 totalSupply = totalSupply();
        _burn(msg.sender, amount);

        for (uint i = 0; i < tokens.length; i++) {
            IERC20 token = tokens[i];

            uint256 preBalance = token.uniBalanceOf(address(this));
            uint256 value = preBalance.mul(amount).div(totalSupply);
            token.uniTransfer(msg.sender, value);
            require(i >= minReturns.length || value >= minReturns[i], "Mooniswap: result is not enough");

            virtualBalancesForAddition[token].scale(preBalance, totalSupply.sub(amount), totalSupply);
            virtualBalancesForRemoval[token].scale(preBalance, totalSupply.sub(amount), totalSupply);
        }

        emit Withdrawn(msg.sender, amount);
    }

    function swap(IERC20 src, IERC20 dst, uint256 amount, uint256 minReturn, address referral) external payable nonReentrant returns(uint256 result) {
        require(msg.value == (src.isETH() ? amount : 0), "Mooniswap: wrong value usage");

        Balances memory balances = Balances({
            src: src.uniBalanceOf(address(this)).sub(src.isETH() ? msg.value : 0),
            dst: dst.uniBalanceOf(address(this))
        });

        // catch possible airdrops and external balance changes for deflationary tokens
        uint256 srcAdditionBalance = Math.max(virtualBalancesForAddition[src].current(balances.src), balances.src);
        uint256 dstRemovalBalance = Math.min(virtualBalancesForRemoval[dst].current(balances.dst), balances.dst);

        src.uniTransferFromSenderToThis(amount);
        uint256 confirmed = src.uniBalanceOf(address(this)).sub(balances.src);
        result = _getReturn(src, dst, confirmed, srcAdditionBalance, dstRemovalBalance);
        require(result > 0 && result >= minReturn, "Mooniswap: return is not enough");
        dst.uniTransfer(msg.sender, result);

        // Update virtual balances to the same direction only at imbalanced state
        if (srcAdditionBalance != balances.src) {
            virtualBalancesForAddition[src].set(srcAdditionBalance.add(confirmed));
        }
        if (dstRemovalBalance != balances.dst) {
            virtualBalancesForRemoval[dst].set(dstRemovalBalance.sub(result));
        }

        // Update virtual balances to the opposite direction
        virtualBalancesForRemoval[src].update(balances.src);
        virtualBalancesForAddition[dst].update(balances.dst);

        if (referral != address(0)) {
            uint256 invariantRatio = uint256(1e36);
            invariantRatio = invariantRatio.mul(balances.src.add(confirmed)).div(balances.src);
            invariantRatio = invariantRatio.mul(balances.dst.sub(result)).div(balances.dst);
            if (invariantRatio > 1e36) {
                // calculate share only if invariant increased
                uint256 referralShare = invariantRatio.sqrt().sub(1e18).mul(totalSupply()).div(1e18).div(REFERRAL_SHARE);
                if (referralShare > 0) {
                    _mint(referral, referralShare);
                }
            }
        }

        emit Swapped(msg.sender, address(src), address(dst), confirmed, result, balances.src, balances.dst, totalSupply(), referral);

        // Overflow of uint128 is desired
        volumes[src].confirmed += uint128(confirmed);
        volumes[src].result += uint128(result);
    }

    function rescueFunds(IERC20 token, uint256 amount) external nonReentrant onlyOwner {
        uint256[] memory balances = new uint256[](tokens.length);
        for (uint i = 0; i < balances.length; i++) {
            balances[i] = tokens[i].uniBalanceOf(address(this));
        }

        token.uniTransfer(msg.sender, amount);

        for (uint i = 0; i < balances.length; i++) {
            require(tokens[i].uniBalanceOf(address(this)) >= balances[i], "Mooniswap: access denied");
        }
        require(balanceOf(address(this)) >= BASE_SUPPLY, "Mooniswap: access denied");
    }

    function _getReturn(IERC20 src, IERC20 dst, uint256 amount, uint256 srcBalance, uint256 dstBalance) internal view returns(uint256) {
        if (isToken[src] && isToken[dst] && src != dst && amount > 0) {
            uint256 taxedAmount = amount.sub(amount.mul(fee()).div(FEE_DENOMINATOR));
            return taxedAmount.mul(dstBalance).div(srcBalance.add(taxedAmount));
        }
    }
}