Insert a row to pandas dataframe

I have a dataframe:

s1 = pd.Series([5, 6, 7])
s2 = pd.Series([7, 8, 9])

df = pd.DataFrame([list(s1), list(s2)],  columns =  ["A", "B", "C"])

   A  B  C
0  5  6  7
1  7  8  9

[2 rows x 3 columns]

and I need to add a first row [2, 3, 4] to get:

   A  B  C
0  2  3  4
1  5  6  7
2  7  8  9

I’ve tried append() and concat() functions but can’t find the right way how to do that.

How to add/insert series to dataframe?