Dear everyone,

I have a dataset as follows.
Basically, in each year, each agency delivers services to some clients.
So, I would like to find how many clients that are new to the agency company in year t+1 compared to year t.
My sample spans from 2003 to 2017.
I am thinking of creating a dummy variable that equals 1 if this client is new to the agency company in year t+1 compared to year t.
However, I actually don't know how to tackle this issue.

I am looking forward for your help.
I really appreciate all your suggestions.
Thank you so much.

Code:
* Example generated by -dataex-. To install: ssc install dataex
clear
input long client double year long agency
109181 2008  2
 64675 2004 12
 21178 2017 13
 13550 2009 16
 13550 2010 16
 13550 2011 16
 13550 2012 16
 13550 2013 16
  9481 2009 17
  9481 2010 17
 31578 2003 20
142541 2003 20
142541 2004 20
 31578 2004 20
 31578 2005 20
 31578 2006 20
162956 2007 20
  4496 2003 21
  4496 2004 21
  4496 2005 21
  4496 2006 21
  4496 2007 21
114834 2008 25
  2581 2003 27
 61877 2003 31
 23153 2003 31
 28285 2003 31
133730 2003 31
 61880 2003 31
 61028 2003 31
 65670 2003 31
 31418 2004 31
 61028 2004 31
 23153 2004 31
 61880 2004 31
 65670 2004 31
 28285 2004 31
133730 2004 31
 61877 2004 31
 61877 2005 31
 61028 2005 31
 25789 2005 31
 23153 2005 31
 31418 2005 31
 61880 2005 31
 31418 2006 31
 60888 2006 31
 23153 2006 31
 25789 2006 31
 61880 2006 31
 61253 2006 31
 61028 2006 31
  5888 2006 31
  5888 2007 31
 31418 2007 31
 60888 2007 31
 61253 2007 31
 23153 2007 31
 61880 2007 31
 25789 2007 31
 61028 2007 31
146071 2007 31
146071 2008 31
 61028 2008 31
 60888 2008 31
 23153 2008 31
 61880 2008 31
 24781 2008 31
  5888 2008 31
 31418 2008 31
 25789 2008 31
 13484 2008 31
 61028 2009 31
 13484 2009 31
 61880 2009 31
 23153 2009 31
 24781 2009 31
 60888 2009 31
 31418 2009 31
  5888 2009 31
 25789 2009 31
122916 2009 31
126020 2004 33
126020 2005 33
 61610 2006 33
126020 2006 33
126020 2007 33
 61610 2007 33
 62046 2008 33
126020 2008 33
 61610 2008 33
126020 2009 33
 61610 2009 33
  8176 2009 33
 31738 2009 38
116903 2010 39
 31738 2010 39
 31738 2011 39
 31738 2012 39
 31738 2013 39
end