library SafeMath {
        /**
         * @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, "Subtractionoverflow.");
                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, "Addition overflow.");

                return c;
        }
}
/**
 * @title SafeMath
 * @dev Removed mul, div, mod
 */
function add(uint256 a, uint256 b) internal pure returns (uint256) {
        uint256 c = a + b;
        require(c >= a, "Addition overflow.");

        return c;
}

/**
 * @dev Adds two unsigned integers, reverts on overflow.
 */