dc, 39
?dsiZdsl[[]r1-d0<m]dsmxklixzll-2%B*C1+P
dc is not known for its string handling, but we have just enough here to get this to work:
?dsi # Read input string, duplicate, store in register i
Zdsl # Get input length, duplicate, store in register l
[ ] # define macro to:
[] # push empty string
r # swap empty string and remaining length
1- # subtract 1 from length
d0 # duplicate and compare with 0
<m # if >0 recursively call this macro again
dsmx # duplicate macro, store in register m and execute
k # discard left-over 0
lix # load input string and execute as macro
z # get stack length
ll- # load string length and subract
2% # mod 2 (result is -ve because difference is -ve)
B* # multiply by 11 ('y' - 'n')
C1+ # add 121 ('y')
P # print result as ASCII char
