Flatten Nested Dictionary

Given a nested dictionary d and a string delimiter, your task is to create a flattened version of the dictionary. In the flattened dictionary, each key is formed by concatenating the keys from the original nested structure, separated by the given delimiter.

For example, given the nested dictionary

{'a': 1, 'b': {'c': 2, 'd': {'e': 3, 'f': 4}}}

and delimiter = "_", the flattened dictionary is

{'a': 1, 'b_c': 2, 'b_d_e': 3, 'b_d_f': 4}

The task can be described mathematically as: \[ \text{flatten}(d, s) = \{\;k\_1 \oplus s \oplus k\_2 \oplus s \oplus \cdots \oplus k\_n : v\}\quad\text{for each nested key path}\; (k_1, k_2, \dots, k_n) \text{ with value } v. \]

Note that if a value is not a dictionary, it remains unchanged.

inputFormat

The input is read from stdin and consists of two lines:

1. A JSON-formatted string representing the nested dictionary.
2. A string representing the delimiter.

Both lines are guaranteed to be non-empty.

outputFormat

The output should be printed to stdout as a JSON-formatted string of the flattened dictionary.

## sample

{"a": 1, "b": {"c": 2, "d": {"e": 3, "f": 4}}}
_

{"a":1,"b_c":2,"b_d_e":3,"b_d_f":4}