using System;

using System.Text.Json;

public class Program

{

    private const string sitesAvailableKey = "sitesAvailability";

    public static void Main()

    {

        string data = "{ \"sitesAvailability\": { \"2020-09-07\": 0.99 } }";

        double availability = GetAvailability(data);

        Console.WriteLine(availability);

    }

    public static double GetAvailability(string data)

    {

        var element = JsonDocument.Parse(data).RootElement.GetProperty(sitesAvailableKey).EnumerateObject();

        element.MoveNext();

        var props = element.Current;

        double availability = props.Value.GetDouble();

        return availability;

    }

}