'+'不能将Fixnum转换成字符串(类型错误)

I've hit upon a 'can't convert Fixnum into String (TypeError)' error and whilst it seems simple enough I'm unsure about how to get around it. I thought my logic was sound - convert the entered string variable to an integer and then carry out the basic operation - but apparently I'm missing some key bit of information.I've hit upon a 'can't convert Fixnum into Stri