library SafeMath {

    /// @dev Multiply two numbers, throw on overflow.
    function mul(uint256 a, uint256 b) internal pure returns (uint256 c) {
        if (a == 0) {
            return 0;
        }
        c = a * b;
        assert(c / a == b);
        return c;
    }

    /// @dev Substract two numbers, throw 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 Add two numbers, throw 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 throw on error
function sub(uint256 a, uint256 b) internal pure returns (uint256) {
        assert(b <= a);
        return a - b;
    }
/// @dev Substract two numbers, throw on overflow (i.e. if subtrahend is greater than minuend).