In a certain river, there are a bunch of stepping stones arranged from one side to the other. A very athletic person can cross the river by jumping on these stepping stones, one at a time.

A stepping stone is big enough for only one person, and the gap between two stepping stones is small enough that it is possible to jump between two adjacent stepping stones.

You are an army commander trying to get a group of soldiers across this river (using these stepping stones). Initially your n soldiers are placed on the first n stepping stones. Your task is to get all of them onto the last n stepping stones.

For example, here are the five possible ways to get a group of two soldiers across a river with five stepping stones:

1) ##---  #-#--  -##--  -#-#-  --##-  --#-#  ---##
2) ##---  #-#--  -##--  -#-#-  -#--#  --#-#  ---##
3) ##---  #-#--  #--#-  -#-#-  --##-  --#-#  ---##
4) ##---  #-#--  #--#-  -#-#-  -#--#  --#-#  ---##
5) ##---  #-#--  #--#-  #---#  -#--#  --#-#  ---##

Let C(k,n) be the number of ways of which n soldiers can cross a river with k stepping stones. In the example, C(5,2) = 5.

Find C(50,12) mod 987654321.