program mypi
      implicit none
c%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%
c%%%%% MPI declarations
      include 'mpif.h'
      integer nproc, rank, p, ierr, rc
c%%%%% problem declarations
      integer myn,n, istart, iend
      double precision h,s,x,glob_pi
      integer i
c%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%
c%%%% start MPI 
c23456
      call MPI_INIT(ierr)
      if (ierr .ne. MPI_SUCCESS) then
         print *,'Error starting MPI program. Terminating.'
         call MPI_ABORT(MPI_COMM_WORLD, rc, ierr)
      end if
c%%%% what is my number (rank+1) and total number of processors 
      call MPI_COMM_RANK(MPI_COMM_WORLD, rank, ierr)
      call MPI_COMM_SIZE(MPI_COMM_WORLD, nproc, ierr)
c
      p = rank + 1
      if (rank.eq.0) then
          write(6,*) 'Number of processors=',nproc 
      endif
c%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%
c%%%%% actual algorithm 

c%%%% set size of subproblem as a constant
      myn = 100
      n = nproc * myn
c%%%
      h = 1.D0/n

c%%% compute the integral using midpoint rule
      s = 0D0
c%%% 
      istart = rank*myn + 1
      iend = p*myn
      do i = istart, iend
         x = i*h - h/2D0
         s = s + 1D0 / (1D0+x*x)
      enddo
      s = s * 4.D0 * h
      write(6,*) 'My partial result for proc =',p,' is=',s

c %%% PARALLEL: must add all values

      call MPI_REDUCE(s,glob_pi,1,MPI_DOUBLE_PRECISION,MPI_SUM,0, 
     $     MPI_COMM_WORLD,ierr) 

c %%% finished
      if (rank.eq.0) then
         write(6,*) 'Finished with ',n,' subitervals. Result=',glob_pi
         write(6,*) 'Error = ',abs( glob_pi-atan(1.0)*4D0 )
      endif 
c%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%
      call MPI_FINALIZE(ierr)

      end