Ann and Bill participate in a math reality show. Each of them is secretly given one positive integer. They know that their numbers are two consecutive numbers ( for instance, Ann’s number is 7, Bill’s number is 6 ). They know only their own number, and they have to guess the number of the other person. Ann and Bill have the following discussion:

+ Ann to Bill: “ I do not know your number”

+ Bill to Ann: “I do not know your number”

+ Ann to Bill: “ Now I know your number! It is a factor of 20”

What is Ann’s number ?