You are given a code:

def main_solution(x):
    return ('urb' + 'fmk')

Can you predict the output of `main_solution("rwauktnkfb")` without writing any code? Please reason and put your final answer in the following json format: {"output": <your output>}, where <your output> should be the final string.
user
{"ref_input": {"x": "rwauktnkfb"}, "ref_output": "urbfmk", "ref_code": "def main_solution(x):\n    return ('urb' + 'fmk')", "funcname": "main_solution"}
rule