Below is an instruction that describes a task. Write a response that appropriately completes the request in Thai. Please answer in Thai.

### Instruction:
There were 18 students assigned in a minibus for a field trip. Eight of these students were boys. On the day of the field trip, the number of girls and boys was the same since some of the girls were not able to join the trip. How many girls were not able to join the field trip?

### Response:
 เด็กชาย 8 คน + เด็กหญิง 8 คน = <<8+8=16>>16 นักเรียนที่เข้าร่วมทริป
ดังนั้น 18 - 16 = <<18-16=2>>2 เด็กหญิงไม่สามารถเข้าร่วมทริปได้