39. Junior high school student Xiao Peng was surprised to find that: his age this year (2018) is exactly the sum of the four digits of his birth year plus 10. He told this discovery to his math teacher, Mr. Wang, who thought for a moment and said: My age this year is also exactly the sum of the four digits of my birth year plus 10. So, Mr. Wang is $\qquad$ years older than Xiao Peng.
Reference answer: 18