Pandas: How can I merge two dataframes? I found (How do I merge two data frames in Python Pandas?), but do not get the expected result.

I have these two CSV files:

# f1.csv
num   ano
76971  1975
76969  1975
76968  1975
76966  1975
76964  1975
76963  1975
76960  1975


and

# f2.csv
num   ano   dou  url
76971  1975 p1   http://exemplo.com/page1
76968  1975 p2   http://exemplo.com/page10
76966  1975 p2   http://exemplo.com/page100


How do I merge these for to get the result given below?

# Expected result
num   ano   dou  url
76971  1975 p1   http://exemplo.com/page1
76969  1975
76968  1975 p2   http://exemplo.com/page10
76966  1975 p2   http://exemplo.com/page100
76964  1975
76963  1975
76960  1975
f1.merge(f2, left_on='num', right_on='num', how='outer')

see https://pandas.pydata.org/pandas-docs/stable/generated/pandas.DataFrame.merge.html