program one2n_write_file_format
      write(*,*)'what is N for 1+2+...N'
      read(*,*) n
      open(unit=10,file='mid.txt')
      m = 0
      do i=1,n
        m=m+i
c       write(10,*)'At stage ',i,' the result is ', m
        write(10,100) i,m
      end do
c     write(*,*)'The final result is',m
      write(*,101)m
      write(*,*)'Intermidiate results are in mid.txt.'
 100  format(1x,'At stage ',i3,' the result is ',i4)
 101  format(1x,'The final result is',i4)
      end