To find the difference, I can subtract any two consecutive terms, for example $7 - 2 = 5$ or $12 - 7 = 5$. So the difference is $5$ and I can use it to find $a$ and $b$. To get $a$, I need to add $5$ to $12$, so $a = 12 + 5 = 17$. To get $b$, I need to add $5$ to $a$, so $b = a + 5 = 17 + 5 = 22$. Therefore, $a + b = 17 + 22 = 39$. 
The sequence $2, 7, 12, a, b, 27$ is arithmetic. What is the value of $a + b$?