How many low-income countries under the lending category of the International Development Associations have a note on the series code SM.POP.TOTL?
low-income countries are where the incomegroup = Low income
SELECT COUNT(T1.Countrycode) FROM Country AS T1 INNER JOIN CountryNotes AS T2 ON T1.CountryCode = T2.Countrycode WHERE T1.LendingCategory = 'IDA' AND T2.Seriescode = 'SM.POP.TOTL' AND IncomeGroup = 'Low income'