Alternating cipher

Suppose RFMTD is encoded to SENSE according to an alternating substitution, how is MATH IS FUN encoded under the same cipher?

Details

• An alternating substitution $M_n$ makes, for instance, the following mapping

$\begin{aligned} M_2(\textrm{a})& \rightarrow \textrm{y} \\ M_2(\textrm{b})& \rightarrow \textrm{d} \\ M_2(\textrm{c})& \rightarrow \textrm{a} \\ M_2(\textrm{d})& \rightarrow \textrm{f} \\ M_2(\textrm{e})& \rightarrow \textrm{c} \\ &\ldots \\ M_2(\textrm{z})& \rightarrow \textrm{b} \end{aligned}$

In general, $n$ can be any integer, positive or negative.