You are given a code:

def main_solution(x):
    return (func_13('tkmdo', '_') + ('rngiug' + 'pzfp'))

Can you predict the output of `main_solution("mjppkumind")` 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": "mjppkumind"}, "ref_output": "t_k_m_d_orngiugpzfp", "ref_code": "def main_solution(x):\n    return (func_13('tkmdo', '_') + ('rngiug' + 'pzfp'))", "funcname": "main_solution"}
rule