You are given a code:

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

Can you predict the output of `main_solution("tbbekgpogf")` 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": "tbbekgpogf"}, "ref_output": "t_b_b_e_k_g_p_o_g_f_n_t_w_c_u_j", "ref_code": "def main_solution(x):\n    return func_13((x + 'ntwcuj'), '_')", "funcname": "main_solution"}
rule