Let $(R,m)$ be a regular local ring of dimension $d$ and char $p>0.$ Let $F^e:R\longrightarrow R$ defined by $r\longrightarrow r^{p^e}$be the Frobenius map.

How to compute $l(R/m^{[p^e]})?.$

I know the answer is $p^{ed}$ but I do not know how to prove it.