Among the countries with note on the series code SM.POP.TOTL, how many of them are in the low-income group?
countries refer to Countrycode; low-income group refers to incomegroup = 'Low income'; with notes refers to description IS NOT NULL; series code SM.POP.TOTL refers to Seriescode = 'SM.POP.TOTL'
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'