$${K = \frac{e^2}{16 \pi \varepsilon_0 R} \\ = \frac{(1,60 \times 10^{-19}\, \mathrm{C})^2}{(16 \pi)(8,85 \times 10^{-12}\, \mathrm{F/m})(1 \times 10^{-15}\, \mathrm{m})} \\ = 5,75 \times 10^{-14}\, \mathrm{J} = 360\, \mathrm{keV} \approx \underline{\underline{400\, \mathrm{keV}}}}$$