Creating a merge_dicts function without explicit key existence check

Write a function merge_dicts that merges two dictionaries: all key-value pairs from the second dictionary are added to the first, but only if the corresponding key does not already exist in the first. It is not recommended to use the if operator to check for the presence of a key during implementation.

import json

def merge_dicts(first_dict: dict, second_dict: dict) -> dict:
    for key, value in second_dict.items():
        first_dict[key] = value
    return first_dict

if __name__ == "__main__":
    first_str, second_str = input().split(';')
    first_dict = json.loads(first_str)
    second_dict = json.loads(second_str)
    print(dict(sorted(merge_dicts(first_dict, second_dict).items())))

The input consists of two JSON strings separated by a semicolon. The program should output the resulting dictionary sorted by keys.