Python Pandas maximum of rows by element

Question

I have a Pandas dataframe like

1 21
1 34
1 25 
2 43
3 32
3 47
4 39
4 31

and I want to find the maximum of all elements by first row element, i.e. I want a new datframe that looks something like this:

1 34
2 43
3 47
4 39

How do I do this?