Swift:不能将类型'int'的值赋给类型'int'的值吗?的错误

I'm starting to learn swift, but ran into an error. Ive just created a very simple app from a course I'm following, that calculates a cats ages based on what the user enters. The first version of the app just times what the user enters by 7, which i managed todo no problem. I thought I would have a play with what I've written and change it todo this:I'm starting to learn swift, but ran into an er