c This program reads integer value n from user and
c calculate 1+2+...+n while write the intermediate resuls
c to a file on disk. The final result is printed on screen.
 
      program one2n_write_file
      write(*,*) 'What is the N for 1 + 2 + ... + N ?'
      read(*,*) n
      open(unit=10,file='detail.txt')
      m = 0
      do i=1, n
        m = m + i
        write(10,*) 'Intermediate result at stage',i,'is',m
      end do
      close(10)
      write(*,*)'The final result is', m
      write(*,*)'Intermedaiate results can be found in detail.txt'
      end