program sum_one2n 
      implicit none
      integer i , sum , n
      write(*,*) 'Calculate 1 + 2 + ...  + N'
      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
      end