integer i, j, n
      double precision u(0:n+1,0:n+1), unew(0:n+1,0:n+1)
      do 10 j=1, n
         do 10 i=1, n
            unew(i,j) = 
     $          0.25*(u(i-1,j)+u(i,j+1)+u(i,j-1)+u(i+1,j)) - 
     $          h * h * f(i,j)
 10   continue