<SYSTEM_TASK:>
Solve the following problem using Python, implementing the functions described below, one line at a time
<END_TASK>
<USER_TASK:>
Description:
 def read_fastas(input_files):
 """ Read the tumor and normal fastas into a joint dict. :param dict input_files: A dict containing filename: filepath for T_ and N_ transgened files. :return: The read fastas in a dictionary of tuples :rtype: dict """

    tumor_file = [y for x, y in input_files.items() if x.startswith('T')][0]
    normal_file = [y for x, y in input_files.items() if x.startswith('N')][0]
    output_files = defaultdict(list)
    output_files = _read_fasta(tumor_file, output_files)
    num_entries = len(output_files)
    output_files = _read_fasta(normal_file, output_files)
    assert len(output_files) == num_entries
    return output_files