how to get sub directories in int-ftp:outbound-gateway with expression?

firstly thanks for attention i have spring integration project that listen ftp server to receive files in different directory and process it,ftp directory structure is:
ftp-root
-----------a\
-----------------in\
---------------------------a.txt
-----------------out\
------------b\
-----------------in\
---------------------------b.txt
-----------------out\

i used int-ftp:outbound-gateway adapter to run ls recursive command,how to write expression attr in int-ftp:outbound-gateway that get a.txt and b.txt in in directories, my adapter code is:

 <int-ftp:outbound-gateway id="gateway1"
                              session-factory="ftpClientFactory"
                              request-channel="inbound1"
                              reply-channel="outbound"
                              reply-timeout="777"
                              auto-create-local-directory="false"
                              auto-startup="true"
                              command="ls"
                              command-options="-1 -R"
                              expression="'*/in/'"
                              mput-regex=".*">

    </int-ftp:outbound-gateway>

but not correctly worked, how to write it?