Error Constant value -1 cannot be converted to a ulong and Operator cannot be applied in C#

Dear Sir,

I have an error `Error CS0221 Constant value '-1' cannot be converted to a 'ulong' (use 'unchecked' syntax to override) and Error CS0019 Operator '>' cannot be applied to operands of type 'bool' and 'bool'`

Please Guide me

public static RColor FromArgb(int alpha, int red, int green, int blue)
{
    RColor.CheckByte(alpha);
    RColor.CheckByte(red);
    RColor.CheckByte(green);
    RColor.CheckByte(blue);
    'below error CS0221 Constant value '-1' cannot be converted to a 'ulong' (use 'unchecked' syntax to override)`
    return new RColor((long)((ulong)(red << 16 | green << 8 | blue | alpha << 24) & (ulong)-1));
}
 
private static string DecodeHtmlCharByCode(string str)
{
    for (int i = str.IndexOf("&#", StringComparison.OrdinalIgnoreCase); i > -1; i = str.IndexOf("&#", i + 1))
    {
        bool flag = str.Length > i + 3 && char.ToLower(str[i + 2]) == 'x';
        'below Error    CS0019  Operator '>' cannot be applied to operands of type 'bool' and 'bool'`
        int num = i + 2 + ((flag > false) ? 1 : 0);
        long num2 = 0L;
        while (num < str.Length && CommonUtils.IsDigit(str[num], flag))
        {
            num2 = num2 * (flag ? 16L : 10L) + (long)CommonUtils.ToDigit(str[num++], flag);
        }
        num += ((num < str.Length && str[num] == ';') ? 1 : 0);
        string value = string.Empty;
        if (num2 >= 0L && num2 <= 1114111L && (num2 < 55296L || num2 > 57343L))
        {
            value = char.ConvertFromUtf32((int)num2);
        }
        str = str.Remove(i, num - i);
        str = str.Insert(i, value);
    }
    return str;
}