library SafeMath {

    /**
    * @dev Multiplies two numbers, throws on overflow.
    */

    function mul(uint256 a, uint256 b) internal pure returns (uint256) {

        if (a == 0) {
            return 0;
        }

        uint256 c = a * b;
        assert(c / a == b);
        return c;

    }

 
    /**
    * @dev Integer division of two numbers, truncating the quotient.
    */

    function div(uint256 a, uint256 b) internal pure returns (uint256) {

        uint256 c = a / b;
        return c;

    }

 
    /**
    * @dev Substracts two numbers, throws on overflow (i.e. if subtrahend is greater than minuend).
    */

    function sub(uint256 a, uint256 b) internal pure returns (uint256) {

        assert(b <= a);
        return a - b;

    }

 
    /**
    * @dev Adds two numbers, throws on overflow.
    */

    function add(uint256 a, uint256 b) internal pure returns (uint256) {

        uint256 c = a + b;
        assert(c >= a);
        return c;

    }

}
function div(uint256 a, uint256 b) internal pure returns (uint256) {

    uint256 c = a / b;
    return c;

}

/**
 * @dev Integer division of two numbers, truncating the quotient.
 */