library SafeMath {
    /**
    * @dev Multiplies two unsigned integers, reverts on overflow.
    */
    function mul(uint256 a, uint256 b) internal pure returns (uint256){
        if (a == 0) {
            return 0;
        }
        uint256 c = a * b;
        require(c / a == b,"Calculation error");
        return c;
    }

    /**
    * @dev Integer division of two unsigned integers truncating the qu
    * otient, reverts on division by zero.
    */
    function div(uint256 a, uint256 b) internal pure returns (uint256){
        // Solidity only automatically asserts when dividing by 0
        require(b > 0,"Calculation error");
        uint256 c = a / b;
        // assert(a == b * c + a % b); // There is no case in which this doesn't hold
        return c;
    }

    /**
    * @dev Subtracts two unsigned integers, reverts on overflow (i.e. if subtrahend is greater than minuend).
    */
    function sub(uint256 a, uint256 b) internal pure returns (uint256){
        require(b <= a,"Calculation error");
        uint256 c = a - b;
        return c;
    }

    /**
    * @dev Adds two unsigned integers, reverts on overflow.
    */
    function add(uint256 a, uint256 b) internal pure returns (uint256){
        uint256 c = a + b;
        require(c >= a,"Calculation error");
        return c;
    }

    /**
    * @dev Divides two unsigned integers and returns the remainder (unsigned integer modulo),
    * reverts when dividing by zero.
    */
    function mod(uint256 a, uint256 b) internal pure returns (uint256){
        require(b != 0,"Calculation error");
        return a % b;
    }
}
/**
 * @title SafeMath
 * @dev Unsigned math operations with safety checks that revert on error
 */
function sub(uint256 a, uint256 b) internal pure returns (uint256){
        require(b <= a,"Calculation error");
        uint256 c = a - b;
        return c;
    }
/**
 * @dev Subtracts two unsigned integers, reverts on overflow (i.e. if subtrahend is greater than minuend).
 */