Use the following rule set. Slater's rules: write the configuration in groups $(1s)(2s,2p)(3s,3p)(3d)(4s,4p)(4d)(4f)(5s,5p)\dots$ For an electron in an $ns$ or $np$ group, every other electron in the same group contributes $0.35$ ($0.30$ for the $1s$ group), every electron in the $(n-1)$ shell contributes $0.85$, every electron in the $(n-2)$ or lower shells contributes $1.00$, and electrons in groups to the right contribute $0$. For an electron in an $nd$ or $nf$ group, every other electron in the same group contributes $0.35$, every electron in all groups to its left contributes $1.00$, and groups to the right contribute $0$. $Z^{\ast }=Z-S$. The Slater orbital energy is $E=-13.6\frac{(Z^{\ast }{)}^2}{(n^{\ast }{)}^2}\mathrm{eV}$ with effective principal quantum number $n^{\ast }=3.7$ for $n=4$. Iron is $(1s{)}^2(2s,2p{)}^8(3s,3p{)}^8(3d{)}^6(4s{)}^2$. Calculate the magnitude of the energy of a $4s$ electron of iron in eV (one decimal place).

Numerical answer type, answer in eV, tolerance 0.1.