program one2n_write_file_format
      write(*,*)'what is N for 1+2+...N'
      read(*,*) n
      open(unit=10,file='mid.txt')
      a = 0
      do i=1,n
        a=a+i
c       write(10,*)'At stage ',i,' the result is ', m
        write(10,100) i,a
      end do
c     write(*,*)'The final result is',m
      write(*,101)a
      write(*,*)'Intermidiate results are in mid.txt.'
 100  format(1x,'At stage ',i3,' the result is ',f8.3)
 101  format(1x,'The final result is',f8.3)
      end