A block is fastened to a horizontal spring. The block is pulled to a distance $x=10 \mathrm{~cm}$ from its equilibrium position (at $x=0$) on a frictionless surface from rest. The energy of the block at $x=5$ $\mathrm{cm}$ is $0.25 \mathrm{~J}$. The spring constant of the spring is ___________ $\mathrm{Nm}^{-1}$