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