A block of mass '$\mathrm{m}$' (as shown in figure) moving with kinetic energy E compresses a spring through a distance $25 \mathrm{~cm}$ when, its speed is halved. The value of spring constant of used spring will be $\mathrm{nE} \,\,\mathrm{Nm}^{-1}$ for $\mathrm{n}=$ _________.