program serial_pi
      implicit none
      integer n, i
      real*8 PI25DT
      parameter (PI25DT = 3.141592653589793238462643d0)
      real*8 pi, h, sum, x
      logical done
      
      done = .false.
      do while ( .not.done)
         print *, "Enter the number of intervals: (0 quits) "
         read(*,*) n
         
         if (n .eq. 0) exit
         
         h   = 1.0d0/n
         sum = 0.0d0
         do i = 1, n 
            x = h * (i - 0.5d0)
            sum = sum + 4.0d0/(1.0d0 + x*x)
         enddo
         pi = h * sum
         
         print *, " pi is approximately ",pi,
         print *, " Error is ", abs(pi - PI25DT)
         
      end do
      
      stop
      end