You are given a code:

def main_solution(x):
    return func_13(('vwr' + 'xei'), '_')

Can you predict the output of `main_solution("ftnudhj")` 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": "ftnudhj"}, "ref_output": "v_w_r_x_e_i", "ref_code": "def main_solution(x):\n    return func_13(('vwr' + 'xei'), '_')", "funcname": "main_solution"}
rule