Leon was a postman. Every day after Leon woke up, he would feed his 37 cats. Leon didn't really want 37 cats, but one by one they had turned up on his doorstep, and Leon had taken them in.

    Every day, after feeding the cats, Leon would collect the mail from the red postboxes in the village and take it home to sort（分类）. “If only I had 37 helpers instead of 37 cats, my job would be so much easier,” Leon thought. Just then, the cats started meowing for their dinner. That gave Leon an idea. That night, Leon stayed up making 37 cat-sized postal backpacks. Each backpack fitted neatly onto a cat's back. Then, he went around to each postbox, putting signs up that read: Postbox closed. Please use the Postal Cat Service.

    The next day, Leon put a backpack on each cat and dropped off the cats around the village. Leon knew that the cats would be happy lazing around all day. He also knew that they would always come home for dinner. Leon spent the day relaxing, waiting for his helpers to return. Leon was right. The cats did return, but they did not bring home many letters. Somehow, the cats had managed to wriggle（蜿蜒而行） out of their backpacks. Or lose their letters. Or get the letters wet. “Cats are just too unreliable（不可靠的）” Leon thought. Just then, there was a scratching（刮擦声） at Leon's door. A dog was waiting to be invited inside. That gave Leon an idea.

若函数$f\left(x\right)=\left(x+1\right)\left(x-a\right)$为偶函数，则a=（     ）