Write SQL query to solve given problem: Among the countries with note on the series code SM.POP.TOTL, how many of them are in the low-income group?. Keep the solution inside sql tag ```sql [SQL-Query] ```
SELECT COUNT(T1.Countrycode) FROM Country AS T1 INNER JOIN CountryNotes AS T2 ON T1.CountryCode = T2.Countrycode WHERE T2.Seriescode = 'SM.POP.TOTL' AND T1.IncomeGroup = 'Low income'