A cafe has a few tables that seat 6 people. There are also a few tall tables that seat 4 people. There are 7 waiters, 1 manager, and 1 cashier present in the cafe. If all the tables are full, the expression 6x + 4y + 7 + 1 + 1 gives the total number of people in the cafe where x represents the number of six-top tables and y represents the number of four-top tables. An equivalent of this expression is .