program Calculates_N 
         implicit none
         integer :: a , sum_N , N
         write(*,*) " Calculate 1 + 2 + ...  + N "
         write(*,*) " Please input is bigger than zero the integer N " 
         a=1
         sum_N=0 
         read (*,*) N
             do a=1 , N   
               sum_N=sum_N+a
             end do
          write(*,*)"The answer of 1 + 2 + ... + " , N , " is" , sum_N
        stop 
        end program Calculates_N