A boy ties a stone of mass 100 g to the end of a 2 m long string and whirls it around in a horizontal plane. The string can withstand the maximum tension of 80 N . If the maximum speed with which the stone can revolve is $\frac{\mathrm{K}}{\pi} \mathrm{rev} / \mathrm{min}$. The value of K is: (Assume the string is massless and unstretchable)