Consider a computing system with 6 frames of physical memory, in which it starts to a process is running. The process references data on the following pages virtual memory: 4, 7, 2, 3, 7, 1, 6, 5, 3, 6, 4, 2, 7, 8, 9, 6, 8, 1, 5, 2, 9, 7

How many page faults will occur if the previous query uses the optimal page replacement algorithm? In case there are more candidates to replace pages, use the FIFO algorithm among candidates to replacing pages.