library SafeMath {
    /**
    * @dev Multiplies two numbers, reverts on overflow.
    */
    function mul(uint256 _a, uint256 _b) internal pure returns (uint256) {
        uint256 c = _a * _b;
        assert(_a == 0 || c / _a == _b);

        return c;
    }

    /**
    * @dev Integer division of two numbers truncating the quotient, reverts on division by zero.
    */
    function div(uint256 _a, uint256 _b) internal pure returns (uint256) {
        uint256 c = _a / _b;
        return c;
    }

    /**
    * @dev Subtracts two numbers, reverts on overflow (i.e. if subtrahend is greater than minuend).
    */
    function sub(uint256 _a, uint256 _b) internal pure returns (uint256) {
        assert(_b <= _a);
        uint256 c = _a - _b;

        return c;
    }

    /**
    * @dev Adds two numbers, reverts on overflow.
    */
    function add(uint256 _a, uint256 _b) internal pure returns (uint256) {
        uint256 c = _a + _b;
        assert(c >= _a);

        return c;
    }
}
/**
 * @title SafeMath
 * @dev Math operations with safety checks that revert on error
 */
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, reverts on division by zero.
 */