subroutine timeStep4(u,n,error)
      implicit none

      double precision u(n,n)
      double precision error
      double precision tmp, diff
      integer n,i,j
!f2py intent(in,out) :: u
!f2py intent(out) :: error
!f2py intent(in) :: n
      error = 0d0
      do j=2,n-1
         do i=2,n-1
            tmp = u(i,j)
            u(i,j) = (4.0d0*(u(i-1,j) + u(i+1,j) + 
     &                       u(i,j-1) + u(i,j+1)) 
     &                     + u(i-1,j-1) + u(i+1,j+1) + 
     &                       u(i+1,j-1) + u(i-1,j+1))/20.0d0
            diff = u(i,j) - tmp
            error = error + diff*diff
         end do
      end do
      error = sqrt(error)
      
      end subroutine timeStep4