That depends if you’re active or not.
If you’re not active, and you just perform simple day-to-day tasks, the average calorie intake for a 17 year old is 1200 calories per day.
If you’re moderately active, and you perform moderate day-to-day tasks/exercise, then you require 1200 - 2,200 calories per day. If this matches you, I’d recommend adding an extra 200 calories to your diet to achieve the average.
If you’re active, then you need 1200 - 2,600 calories per day. In this case, you’re not eating enough calories, which will stunt body developent.
Also, since you’re underweight, I’d recommend going well over the recommended calories for a while until you achieve average weight, which is 45.3 KG/99.86 lbs.