program one_to_n_sub
      integer sum, i ,n
      write(*,*) 'Please tell me the number N:'
      read(*,*) n
      call adding(sum,n)
      open (unit=10, file='answer.txt')
      write(10,*) 'The answer for 1 + 2 + 3 + ... + N is :',sum
      write(*,*) 'The answer has been written into file answer.txt .'
      end

      subroutine adding(total,n)
      integer total, n, i
      total = 0
      do i=1,n
        total = total + i
      end do
      end