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) {
    //     // Solidity only automatically asserts when dividing by 0
    //     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;
    // }
}
/**
 * @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.
 */
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 addition of two unsigned integers, reverting on
 * overflow.
 *
 * Counterpart to Solidity's `+` operator.
 *
 * Requirements:
 * - Addition cannot overflow.
 */