Benchmark Contest:Count character ratio in DNA file

RULES:
The program must count(without prior knowledge of file beyond its format) GC-content of source file.
GC-content is fraction of GC vs AT chars found in the file expressed as floating point number. http://en.wikipedia.org/wiki/GC-content
source file https://github.com/dubst3pp4/GC-Content-OOC/blob/master/Homo_sapiens.GRCh37.67.dna_rm.chromosome.Y.fa

Why limit to only 4 characters? Why not formulate this problem as a more general problem of the frequency of characters from some arbitrary subset in a stream of arbitrary characters? Do you idiots think you're smarter just for using the word "DNA", or do you think that making solutions ungeneral somehow adds to their "practicality"?