program sumitrust2019B
	integer(8)::N,i
	integer(8)::flg=0
	read(5,*)N
	
	do i=1,N
		if(int(real(i)*1.08)==N)then
			flg=i
			exit
		end if
	end do

	if(flg==0)then
		print'(A)',":("
	else
		print'(i0)',flg
	end if
end program sumitrust2019B
