program ask_n_write_one2n 
      implicit none
      integer n , i
      write(*,*) 'This code writes N and numers 1 to N into a file
     & "data.dat".'
      write(*,*) ' '
      write(*,*) "What is N ?"
      read(*,*) n
      open (unit=20 ,file='data.dat')
      write(20,*) n
      do i = 1, n
      write(20,*) i
      end do
      close(20)
      write(*,*)'Now', n, 'and 1, 2, ..', n, 'is saved in a file
     & "data,dat".' 
      end