program sum_one2n_cycle 
      implicit none
      integer i, sum, n, i_again
      write(*,*) 'Calculate 1 + 2 + ...  + N'
 100  write(*,*) 'Please input N (N must be > or = 1) :' 
      read (*,*) n
      sum=0 
        do i=1, n   
          sum = sum + i
        end do
      write(*,*)'The answer of 1 + 2 + ... + ', n , " is" , sum
      write(*,*)

 101  write(*,*) 'Again ? (1=Yes,0=No) pleasetype in 1 or 0 :'
      write(*,*)
      read(*,*) i_again
      if (i_again.eq.1) goto 100
      if (i_again.eq.0) stop
      goto 101

      end