若a﹣b=1，ab=3，则代数式（a+1）（b﹣1）的值为____ 

Mr. Johnson worked in a hospital. Three years ago, when he was sixty-eight, he retired. But a lot of people come to ask him to look them over. He's very busy in the daytime and he wants to have a good rest at night.
Half a year ago a young man moved in the room upstairs. At first he was quiet. The old man was satisfied with him. But now he has several friends. They often came to see him in the evening. They talked loudly, sang and danced. Mr Johnson was unhappy, but he didn't say anything.
It was the young man's birthday yesterday. All his friends came at seven in the evening. They drank too much and made a lot of noise. The old man couldn't fall asleep the whole night. This morning, as soon as he got up, he went upstairs and began to knock at the door. The young man opened for him and asked him to come in.
"Did you hear me beat the ceiling(天花板) last night, young man?" asked Mr Johnson.
"It doesn't matter, sir," said the young man, "I don't mind it. It was very noisy in my room, too."