program one2n_repeat
 100  write(*,*)'What is your N?'
      read(*,*) n
      m=0
      do i=1,n
        m=m+i
      enddo
      write(*,*)'The answer of 1+...+n is',m
      write(*,*)'Again? (1=yes,0=no)'
      read(*,*) key
      if(key==1)goto 100
      write(*,*)'Bye!'
      end