library SafeMath {

    /**
    * @dev Multiplies two numbers
    */
    function mul(uint a, uint b) internal pure returns (uint) {
        if (a == 0) {
            return 0;
        }
        uint c = a * b;
        require(c / a == b, "mul failed");
        return c;
    }

    /**
    * @dev Subtracts two numbers
    */
    function sub(uint a, uint b) internal pure returns (uint) {
        require(b <= a, "sub failed");
        return a - b;
    }

    /**
    * @dev Adds two numbers
    */
    function add(uint a, uint b) internal pure returns (uint) {
        uint c = a + b;
        require(c >= a, "add failed");
        return c;
    }

    /**
     * @dev Gives square root from number
     */
    function sqrt(uint x) internal pure returns (uint y) {
        uint z = add(x, 1) / 2;
        y = x;
        while (z < y) {
            y = z;
            z = add(x / z, z) / 2;
        }
    }
}
function add(uint a, uint b) internal pure returns (uint) {
    uint c = a + b;
    require(c >= a, "add failed");
    return c;
}

/**
 * @dev Adds two numbers
 */