This is the fifth option. Finally, we want to know the total number of students appearing for at least one course. This can be done using an outer join:

dfSE = pd.concat([df1SE, df2SE], ignore_index=True)
dfML = pd.concat([df1ML, df2ML], ignore_index=True)

df = dfSE.merge(dfML, how='outer')
df

Check the output and compare the differences with the previous output.