pyspark recover for an even number the two values ​of a median

Question

Is there a way i pyspark to recover for an even number the two values ​​of a median ?

For exemple: I have this dataframe

df1 = spark.createDataFrame([
    ('c',1),
    ('c',2),
    ('c',4),
    ('c',6),
    ('c',7),
    ('c',8),
], ['a', 'b'])
df1.show()
+---+---+
|  a|  b|
+---+---+
|  c|  1|
|  c|  2|
|  c|  4|
|  c|  6|
|  c|  7|
|  c|  8|

I want for this dataframe to display the values 4 and 6