DS-1000 / 39 / I have a dataframe with numerous columns (≈30) from an external source (csv…

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Problem:
I have a dataframe with numerous columns (≈30) from an external source (csv file) but several of them have no value or always the same. Thus, I would to see quickly the value_counts for each column. How can i do that?
For example
  id, temp, name
1 34, null, mark
2 22, null, mark
3 34, null, mark


Please return a Series like this:


id    22      1.0
      34      2.0
temp  null    3.0
name  mark    3.0
dtype: float64


So I would know that temp is irrelevant and name is not interesting (always the same)


A:
<code>
import pandas as pd


df = pd.DataFrame(data=[[34, 'null', 'mark'], [22, 'null', 'mark'], [34, 'null', 'mark']], columns=['id', 'temp', 'name'], index=[1, 2, 3])
</code>
result = ... # put solution in this variable
BEGIN SOLUTION
<code>