program prod
      include 'mpif.h'
   
      integer max
      parameter (max=30000)
    
      integer noprocs, nid, i, n, size, error
      integer status(MPI_STATUS_SIZE)
      real a(0:max-1), b(0:max-1), sum, Gsum
   
      call MPI_Init(error)
      call MPI_Comm_rank(MPI_COMM_WORLD, nid, error)
      call MPI_Comm_size(MPI_COMM_WORLD, noprocs, error)
      if (nid .eq. 0) then
         open(9,file='DotData.Txt',form='formatted')
         read(9,*)n
         call MPI_Bcast(n,1,MPI_INTEGER,0,MPI_COMM_WORLD,error)
         if (n .gt. max) then
            write(6,*) ('Need to increase dimension of arrays a and b!')
            call MPI_Abort(MPI_COMM_WORLD,-1,error)
         end if
         if (mod(n,noprocs) .ne. 0) then
            write(6,*) ('Number of processes is not a multiple of n.')
            call MPI_Abort(MPI_COMM_WORLD,-1,error)
         end if
         do 10 i=0,n-1
            read(9,*)a(i),b(i)
   10    continue
         close(9)
         size = n / noprocs
         do 20 i=1,noprocs-1
            call MPI_Send(a(size*i),size,MPI_REAL,i,10,MPI_COMM_WORLD,
     &         error)
            call MPI_Send(b(size*i),size,MPI_REAL,i,20,MPI_COMM_WORLD,
     &         error)
   20    continue
      else
         call MPI_Bcast(n,1,MPI_INTEGER,0,MPI_COMM_WORLD,error)
         if (n .gt. max .or. mod(n,noprocs) .ne. 0) then
            call MPI_Abort(MPI_COMM_WORLD,-1,error)
         end if
         size =  n / noprocs
         call MPI_Recv(a(0),size,MPI_REAL,0,10,MPI_COMM_WORLD,status,
     &      error)
         call MPI_Recv(b(0),size,MPI_REAL,0,20,MPI_COMM_WORLD,status,
     &      error)
      end if
      sum = 0.0
      do 30 i=0,size-1
         sum = sum + a(i) * b(i)
   30 continue
      call MPI_Reduce(sum,Gsum,1,MPI_REAL,MPI_SUM,0,MPI_COMM_WORLD,
     &   error)
      if (nid .eq. 0) then
         write(6,*)'The inner product is ',Gsum
      end if
      call MPI_Finalize(error)
   
      stop
      end