Given the following direct runoff hydrograph in m$3/$s taken at $2-$hr interval in the mainstream of a $80$ sq$.$ km watershed: $0,13\mathrm{.}8,77\mathrm{.}8,12\mathrm{.}8,1\mathrm{.}8,0\mathrm{.}9,0\mathrm{.}2.$ The volume of direct runoff is$?$