program one2n_write_file_2
      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
        write(10,*) m
      end do
      write(*,*)'The final result is',m
      write(*,*)'Intermidiate results are in mid.txt.'
      end