How to find unknown value if idf is given

We know the formula for idf is as fol:-

idf = log10 (N/df)

where N is the total number of documents. what if we have idf and df values given and we have to find N. I tried following method but it returns wrong result:-

 if y = logb (开发者_运维知识库x) then
    x = b^y

please help.