Group People by City

You are given a list of people. Each person is described by a name, an age, and optionally a city. Your task is to group the people by their city. If the city field is missing for any person, place that person in a group with the key Unknown.

The output should be a JSON object where each key is a city name and the corresponding value is an array of the names of the people from that city.

Note: In the output, use the exact key Unknown for entries without a provided city.

The algorithm can be expressed mathematically as follows. Let \(P\) be the set of people, and for each person \(p \in P\) define the city as

[ city(p) = \begin{cases} p.city, & \text{if } p.city \text{ is provided},\ \text{Unknown}, & \text{otherwise.} \end{cases} ]

Then, we want to compute the mapping:

[ G = { c : [p.name \mid p \in P,\ city(p)=c] \mid c \in \text{all cities}}. ]

</p>

inputFormat

The input is read from standard input (stdin) in the following format:

• The first line contains an integer n representing the number of people.
• Each of the next n lines contains information for one person in the following format:
  name age [city]
  Here, name is a string, age is a number, and city (without spaces) is optional. If city is missing, treat it as Unknown.

outputFormat

Output a JSON object to standard output (stdout) where the keys are the city names and the values are arrays of names of the people belonging to that city.

## sample

3
Alice 30 New_York
Bob 25 San_Francisco
Charlie 35 New_York

{"New_York": ["Alice", "Charlie"], "San_Francisco": ["Bob"]}