$KS = \dfrac{\left(\dfrac{\cancel{2} \cdot IR}{\cancel{2}}\right)+1}{\left(\dfrac{\cancel{2} \cdot FR}{\cancel{2}}\right)+1}$
$KS = \dfrac{IR+1}{FR+1}$
$KS(FR+1) = \dfrac{IR+1}{\cancel{FR+1}} \cdot (\cancel{FR+1})$
$KS(FR+1) = IR+1$
$\color{red}{KS(FR+1) - 1 = IR}$