To change the volume of a gas isothermally by a factor \(f_{1}\) of its initial value, it is necessary to do work \(W_{1}\) on the gas. What work \(W_{2}\) is needed to change the volume by a factor \(f_{2}\) of the initial gas volume at the same temperature? Express your answer in terms of \(f_{1}, f_{2}\), and \(W_{1}\).