You build a musical score by writing notes one after another. Each note has one of two pitches, low or high, and one of two durations, short or long. A short note lasts 1 second and a long note lasts 2 seconds. The score must satisfy all of the following.
a) The total duration of the score is a given integer n seconds.
b) The number of low short notes equals the number of high short notes.
c) The number of low long notes equals the number of high long notes.
d) The number of long notes is at least the number of short notes.
e) Low notes and high notes alternate.
f) The first note is low.
Given an even integer n, count the scores that satisfy every condition. Writing each note as its duration, the 4 scores for n=6 are 2211, 2112, 1221 and 1122. Conditions e and f force every odd numbered note to be low and every even numbered note to be high. For n=8 the only score is 2222.