def find_files(folder):
    files = [i for i in os.listdir(folder) if i.startswith("left")]
    files.sort()
    for i in range(len(files)):
        insert_string = "right{}".format(files[i * 2][4:])
        files.insert(i * 2 + 1, insert_string)
    files = [os.path.join(folder, filename) for filename in files]
    return files