小时候/乡愁是一枚小小的邮票/我在这头/母亲在那头

    长大后/乡愁是一张窄窄的船票/我在这头/新娘在那头

    后来啊/乡愁是一方矮矮的坟墓/我在外头/母亲在里头

    而现在/乡愁是一湾浅浅的海峡/我在这头/大陆在那头

    One day an old man is selling a big elephant. A young man comes to the elephant and begins to look at it slowly. The old man goes up to him and says in his ear, “Don't say anything about the elephant before I sell it. Then I will give you twenty dollars.” “All right,” says the young man. After the old man sells the elephant, he gives the young man twenty dollars and says, “Now, can you tell me how you found the elephant's bad ears?” “I didn't find the bad ears,” says the young man. “Then why are you looking at the elephant slowly?” asked the old man. The young man answers, “Because I have never seen an elephant before, and I want to know what it looks like.”

若关于x的不等式$\left\{\begin{array}{c}x-a\ge 0\\ 2x-b<0\end{array}\right.$的整数解为x=1,x=2.则适合这个不等式组的整数a、b的有序实数对（a,b）共有(     )